There are a lot of scripts out there that will do adsense style ads.
Just google - text ad script -
SonnyCooL (8 March, 2010)
All scripts you can find online are scripts you can find exploits for. So I wouldn't go for an online paid or downloadable script, if you want something good, you should script it yourself or outsource the scripting -- it'll cost a lot, but if you will create something worth using it will pay back
take a look at this adbrite clone script
phplemon - AdBrite Clone Ad Server PHP Script - "Get Paid To Post" PHP Script