[ForumDog]

I have done this for our forum, it replicates the Digg rating system for not just posts but threads as well, with one or two minor improvements over the Digg system, probably most significantly that my version has three levels of user-definable settings (Digg may do this, I've never actually used their site). You can enable ratings and post visibility based on ratings, or set it so that ratings will show up but posts & threads will not be visibly affected but meaning that you can't place votes yourself, or disable the whole lot and no ratings or effects will appear at all. See here for a not-quite-thorough explanation.

I can't release the code because:
a) It's customised code from a customised forum. Not really suitable for releasing as a mod for a default forum without some very major surgery.
b) It's nice to have a feature that others don't have (as far as I know) so I'd rather leave it to attract the curious and send a visitor or two our way. Feel free, by the way.
c) It's based on someone else's code, named the Post Thanks and Post Groan hacks so a substantial amount of the work & credit lies with the author. So if you have the appropriate skills and you're willing to spend the time making the adaptions & changes, that would be a good place to start. The thread Digging part also required changes to thread ratings as well as both the plugins - though it was a borderline cheat - and obviously template changes are very significant in the appropriate places.

It was not an insignificant amount of work, though working with someone else's code and being the first to do the experimentation without knowing precisely what to do bumped up the development time quite a bit I expect.