This is a grey tactic that needs to be done right/be relevant to sites links are dropped in for it to be effective (and not get deleted from said sites). When it comes to finding places to drop your link, google is an awesome tool. The formula is pretty much like this:
- Go to hotscripts.com or similar collection of scripts and find scripts that allow user contribution in some way & the posting of a dofollow link (in the form of comments, contributions, profiles etc). Script examples can be forums, blogs (blog posts as well, not just comments), user content powered sites, wikis, shoutboxes, guestbooks, and a large number of all kinds of CMS that allow for creation of a profile or adding content in some way.
- Now google a footprint (can be copyright footer, standard file name, default title, any piece of text that is usually left unchanged by webmasters) unique to the script of your selection + your keyword. (Such as "powered by pligg ufo" or "inurl:showthread.php tutorial" or "allintitleowered by vbulletin your keyword)
- Happy link dropping, best done manually to make it look least spammy in the eyes of site webmasters & avoid duplicate content filters. This can be outsourced to persons who do data entry type jobs, and you can split different scripts among different people if you are particular about keeping this stuff under the covers.