robots.txt is somewhat useless because really bad robots just ignore it. Still, some robots do pay attention to directives in robots.txt, so it does save some bandwidth and annoyance.

I am pretty severe with my robots.txt. Here are all of the User-agents I am currently denying:
Code:
User-agent: OutfoxBot/0.5
User-agent: complex_network_group
User-agent: Alexibot
User-agent: Aqua_Products
User-agent: BackDoorBot
User-agent: BackDoorBot/1.0
User-agent: BPImageWalker/2.0
User-agent: Black.Hole
User-agent: BlackWidow
User-agent: BlowFish
User-agent: BlowFish/1.0
User-agent: Bookmark search tool
User-agent: Bot mailto:craftbot@yahoo.com
User-agent: BotALot
User-agent: BotRightHere
User-agent: BuiltBotTough
User-agent: Bullseye
User-agent: Bullseye/1.0
User-agent: BunnySlippers
User-agent: Cegbfeieh
User-agent: CheeseBot
User-agent: CherryPicker
User-agent: CherryPickerElite/1.0
User-agent: CherryPickerSE/1.0
User-agent: ChinaClaw
User-agent: Copernic
User-agent: CopyRightCheck
User-agent: Crescent
User-agent: Crescent Internet ToolPak HTTP OLE Control v.1.0
User-agent: Custo
User-agent: DISCo
User-agent: DISCo Pump 3.0
User-agent: DISCo Pump 3.2
User-agent: DISCoFinder
User-agent: DittoSpyder
User-agent: Download Demon
User-agent: Download Demon/3.2.0.8
User-agent: Download Demon/3.5.0.11
User-agent: EirGrabber
User-agent: EmailCollector
User-agent: EmailSiphon
User-agent: EmailWolf
User-agent: EroCrawler
User-agent: Express WebPictures
User-agent: Express WebPictures (www.express-soft.com)
User-agent: ExtractorPro
User-agent: EyeNetIE
User-agent: FairAd Client
User-agent: Flaming AttackBot
User-agent: FlashGet
User-agent: FlashGet WebWasher 3.2
User-agent: Foobot
User-agent: FrontPage
User-agent: FrontPage [NC,OR]
User-agent: Gaisbot
User-agent: GetRight
User-agent: GetRight/2.11
User-agent: GetRight/3.1
User-agent: GetRight/3.2
User-agent: GetRight/3.3
User-agent: GetRight/3.3.3
User-agent: GetRight/3.3.4
User-agent: GetRight/4.0.0
User-agent: GetRight/4.1.0
User-agent: GetRight/4.1.1
User-agent: GetRight/4.1.2
User-agent: GetRight/4.2
User-agent: GetRight/4.2b (Portuguxeas)
User-agent: GetRight/4.2c
User-agent: GetRight/4.3
User-agent: GetRight/4.5
User-agent: GetRight/4.5a
User-agent: GetRight/4.5b
User-agent: GetRight/4.5b1
User-agent: GetRight/4.5b2
User-agent: GetRight/4.5b3
User-agent: GetRight/4.5b6
User-agent: GetRight/4.5b7
User-agent: GetRight/4.5c
User-agent: GetRight/4.5d
User-agent: GetRight/4.5e
User-agent: GetRight/5.0beta1
User-agent: GetRight/5.0beta2
User-agent: GetWeb!
User-agent: Go!Zilla
User-agent: Go!Zilla (www.gozilla.com)
User-agent: Go!Zilla 3.3 (www.gozilla.com)
User-agent: Go!Zilla 3.5 (www.gozilla.com)
User-agent: Go-Ahead-Got-It
User-agent: GrabNet
User-agent: Grafula
User-agent: HMView
User-agent: HTTrack
User-agent: HTTrack 3.0
User-agent: HTTrack 3.0x
User-agent: HTTrack [NC,OR]
User-agent: Harvest
User-agent: Harvest/1.5
User-agent: Image Stripper
User-agent: Image Sucker
User-agent: Indy Library
User-agent: Indy Library [NC,OR]
User-agent: InfoNaviRobot
User-agent: InterGET
User-agent: Internet Ninja
User-agent: Internet Ninja 4.0
User-agent: Internet Ninja 5.0
User-agent: Internet Ninja 6.0
User-agent: Iron33/1.0.2
User-agent: JOC Web Spider
User-agent: JennyBot
User-agent: JetCar
User-agent: Kenjin Spider
User-agent: Kenjin.Spider
User-agent: Keyword Density/0.9
User-agent: Keyword.Density
User-agent: LNSpiderguy
User-agent: LeechFTP
User-agent: LexiBot
User-agent: LinkScan/8.1a Unix
User-agent: LinkScan/8.1a.Unix
User-agent: LinkWalker
User-agent: LinkextractorPro
User-agent: MIDown tool
User-agent: MIIxpc
User-agent: MIIxpc/4.2
User-agent: MSIECrawler
User-agent: Mass Downloader
User-agent: Mass Downloader/2.2
User-agent: Mata Hari
User-agent: Mata.Hari
User-agent: Microsoft URL Control
User-agent: Microsoft URL Control - 5.01.4511
User-agent: Microsoft URL Control - 6.00.8169
User-agent: Microsoft.URL
User-agent: Mister PiX
User-agent: Mister PiX version.dll
User-agent: Mister Pix II 2.01
User-agent: Mister Pix II 2.02a
User-agent: Mister.PiX
User-agent: NICErsPRO
User-agent: NPBot
User-agent: NPbot
User-agent: Navroad
User-agent: NearSite
User-agent: Net Vampire
User-agent: Net Vampire/3.0
User-agent: NetAnts
User-agent: NetAnts/1.10
User-agent: NetAnts/1.23
User-agent: NetAnts/1.24
User-agent: NetAnts/1.25
User-agent: NetMechanic
User-agent: NetSpider
User-agent: NetZIP
User-agent: NetZip Downloader 1.0 Win32(Nov 12 1998)
User-agent: NetZip-Downloader/1.0.62 (Win32; Dec 7 1998)
User-agent: NetZippy+(http:/www.innerprise.net/usp-spider.asp)
User-agent: Octopus
User-agent: Offline Explorer
User-agent: Offline Explorer/1.2
User-agent: Offline Explorer/1.4
User-agent: Offline Explorer/1.6
User-agent: Offline Explorer/1.7
User-agent: Offline Explorer/1.9
User-agent: Offline Explorer/2.0
User-agent: Offline Explorer/2.1
User-agent: Offline Explorer/2.3
User-agent: Offline Explorer/2.4
User-agent: Offline Explorer/2.5
User-agent: Offline Navigator
User-agent: Offline.Explorer
User-agent: Openbot
User-agent: Openfind
User-agent: Openfind data gatherer
User-agent: Oracle Ultra Search
User-agent: PageGrabber
User-agent: Papa Foto
User-agent: PerMan
User-agent: ProPowerBot/2.14
User-agent: ProWebWalker
User-agent: Python-urllib
User-agent: QueryN Metasearch
User-agent: QueryN.Metasearch
User-agent: RMA
User-agent: Radiation Retriever 1.1
User-agent: ReGet
User-agent: RealDownload
User-agent: RealDownload/4.0.0.40
User-agent: RealDownload/4.0.0.41
User-agent: RealDownload/4.0.0.42
User-agent: RepoMonkey
User-agent: RepoMonkey Bait & Tackle/v1.01
User-agent: SiteSnagger
User-agent: SlySearch
User-agent: SmartDownload
User-agent: SmartDownload/1.2.76 (Win32; Apr 1 1999)
User-agent: SmartDownload/1.2.77 (Win32; Aug 17 1999)
User-agent: SmartDownload/1.2.77 (Win32; Feb 1 2000)
User-agent: SmartDownload/1.2.77 (Win32; Jun 19 2001)
User-agent: SpankBot
User-agent: Sqworm/2.9.85-BETA (beta_release; 20011115-775; i686-pc-linux
User-agent: SuperBot
User-agent: SuperBot/3.0 (Win32)
User-agent: SuperBot/3.1 (Win32)
User-agent: SuperHTTP
User-agent: SuperHTTP/1.0
User-agent: Surfbot
User-agent: Szukacz/1.4
User-agent: Teleport
User-agent: Teleport Pro
User-agent: Teleport Pro/1.29
User-agent: Teleport Pro/1.29.1590
User-agent: Teleport Pro/1.29.1634
User-agent: Teleport Pro/1.29.1718
User-agent: Teleport Pro/1.29.1820
User-agent: Teleport Pro/1.29.1847
User-agent: TeleportPro
User-agent: Telesoft
User-agent: The Intraformant
User-agent: The.Intraformant
User-agent: TheNomad
User-agent: TightTwatBot
User-agent: Titan
User-agent: True_Robot
User-agent: True_Robot/1.0
User-agent: TurnitinBot
User-agent: TurnitinBot/1.5
User-agent: URL Control
User-agent: URL_Spider_Pro
User-agent: URLy Warning
User-agent: URLy.Warning
User-agent: VCI
User-agent: VCI WebViewer VCI WebViewer Win32
User-agent: VoidEYE
User-agent: WWW-Collector-E
User-agent: WWWOFFLE
User-agent: Web Image Collector
User-agent: Web Sucker
User-agent: Web.Image.Collector
User-agent: WebAuto
User-agent: WebAuto/3.40 (Win98; I)
User-agent: WebBandit
User-agent: WebBandit/3.50
User-agent: WebCapture 2.0
User-agent: WebCopier
User-agent: WebCopier v.2.2
User-agent: WebCopier v2.5
User-agent: WebCopier v2.6
User-agent: WebCopier v2.7a
User-agent: WebCopier v2.8
User-agent: WebCopier v3.0
User-agent: WebCopier v3.0.1
User-agent: WebCopier v3.2
User-agent: WebCopier v3.2a
User-agent: WebEMailExtrac.*
User-agent: WebEnhancer
User-agent: WebFetch
User-agent: WebGo IS
User-agent: WebLeacher
User-agent: WebReaper
User-agent: WebReaper [info@webreaper.net]
User-agent: WebReaper [webreaper@otway.com]
User-agent: WebReaper v9.1 - www.otway.com/webreaper
User-agent: WebReaper v9.7 - www.webreaper.net
User-agent: WebReaper v9.8 - www.webreaper.net
User-agent: WebReaper vWebReaper v7.3 - www,otway.com/webreaper
User-agent: WebSauger
User-agent: WebSauger 1.20b
User-agent: WebSauger 1.20j
User-agent: WebSauger 1.20k
User-agent: WebStripper
User-agent: WebStripper/2.03
User-agent: WebStripper/2.10
User-agent: WebStripper/2.12
User-agent: WebStripper/2.13
User-agent: WebStripper/2.15
User-agent: WebStripper/2.16
User-agent: WebStripper/2.19
User-agent: WebWhacker
User-agent: WebZIP
User-agent: WebZIP/2.75 (http:/www.spidersoft.com)
User-agent: WebZIP/3.65 (http:/www.spidersoft.com)
User-agent: WebZIP/3.80 (http:/www.spidersoft.com)
User-agent: WebZIP/4.0 (http:/www.spidersoft.com)
User-agent: WebZIP/4.1 (http:/www.spidersoft.com)
User-agent: WebZIP/4.21
User-agent: WebZIP/4.21 (http:/www.spidersoft.com)
User-agent: WebZIP/5.0
User-agent: WebZIP/5.0 (http:/www.spidersoft.com)
User-agent: WebZIP/5.0 PR1 (http:/www.spidersoft.com)
User-agent: WebZip
User-agent: WebZip/4.0
User-agent: WebmasterWorldForumBot
User-agent: Website Quester
User-agent: Website Quester - www.asona.org
User-agent: Website Quester - www.esalesbiz.com/extra/
User-agent: Website eXtractor
User-agent: Website eXtractor (http:/www.asona.org)
User-agent: Website.Quester
User-agent: Webster Pro
User-agent: Webster.Pro
User-agent: Wget
User-agent: Wget/1.10.2
User-agent: Wget/1.5.2
User-agent: Wget/1.5.3
User-agent: Wget/1.6
User-agent: Wget/1.7
User-agent: Wget/1.8
User-agent: Wget/1.8.1
User-agent: Wget/1.8.1+cvs
User-agent: Wget/1.8.2
User-agent: Wget/1.9-beta
User-agent: Widow
User-agent: Xaldon WebSpider
User-agent: Xaldon WebSpider 2.5.b3
User-agent: Xenu's
User-agent: Xenu's Link Sleuth 1.1c
User-agent: Zeus
User-agent: Zeus 11389 Webster Pro V2.9 Win32
User-agent: Zeus 11652 Webster Pro V2.9 Win32
User-agent: Zeus 18018 Webster Pro V2.9 Win32
User-agent: Zeus 26378 Webster Pro V2.9 Win32
User-agent: Zeus 30747 Webster Pro V2.9 Win32
User-agent: Zeus 32297 Webster Pro V2.9 Win32
User-agent: Zeus 39206 Webster Pro V2.9 Win32
User-agent: Zeus 41641 Webster Pro V2.9 Win32
User-agent: Zeus 44238 Webster Pro V2.9 Win32
User-agent: Zeus 51070 Webster Pro V2.9 Win32
User-agent: Zeus 51674 Webster Pro V2.9 Win32
User-agent: Zeus 51837 Webster Pro V2.9 Win32
User-agent: Zeus 63567 Webster Pro V2.9 Win32
User-agent: Zeus 6694 Webster Pro V2.9 Win32
User-agent: Zeus 71129 Webster Pro V2.9 Win32
User-agent: Zeus 82016 Webster Pro V2.9 Win32
User-agent: Zeus 82900 Webster Pro V2.9 Win32
User-agent: Zeus 84842 Webster Pro V2.9 Win32
User-agent: Zeus 90872 Webster Pro V2.9 Win32
User-agent: Zeus 94934 Webster Pro V2.9 Win32
User-agent: Zeus 95245 Webster Pro V2.9 Win32
User-agent: Zeus 95351 Webster Pro V2.9 Win32
User-agent: Zeus 97371 Webster Pro V2.9 Win32
User-agent: Zeus Link Scout
User-agent: asterias
User-agent: b2w/0.1
User-agent: cosmos
User-agent: eCatch
User-agent: eCatch/3.0
User-agent: hloader
User-agent: httplib
User-agent: humanlinks
User-agent: larbin
User-agent: larbin (samualt9@bigfoot.com)
User-agent: larbin samualt9@bigfoot.com
User-agent: larbin_2.6.2 (kabura@sushi.com)
User-agent: larbin_2.6.2 (larbin2.6.2@unspecified.mail)
User-agent: larbin_2.6.2 (listonATccDOTgatechDOTedu)
User-agent: larbin_2.6.2 (vitalbox1@hotmail.com)
User-agent: larbin_2.6.2 kabura@sushi.com
User-agent: larbin_2.6.2 larbin2.6.2@unspecified.mail
User-agent: larbin_2.6.2 larbin@correa.org
User-agent: larbin_2.6.2 listonATccDOTgatechDOTedu
User-agent: larbin_2.6.2 vitalbox1@hotmail.com
User-agent: libWeb/clsHTTP
User-agent: lwp-trivial
User-agent: lwp-trivial/1.34
User-agent: moget
User-agent: moget/2.1
User-agent: pavuk
User-agent: pcBrowser
User-agent: psbot
User-agent: searchpreview
User-agent: spanner
User-agent: suzuran
User-agent: tAkeOut
User-agent: toCrawl/UrlDispatcher
User-agent: turingos
User-agent: webfetch/2.1.0
User-agent: wget
Disallow: /
This list is obviously out of date. Updating it is another item on the ToDo list.