A lot of places, mainly online stores, now have the 'if you like this you may like...' or 'customers who bought this also bought...' features to help (and encourage) you to find similar items you might like. Unfortunately they're often not very good, people buy a lot of wild things on one order so you get some strange results.
I was thinking, not along ago, of adding a section to my film blog where you could add recommendations for similar films (I forget what I had just watched which had such a resonance with another film). I had gone looking for such a site, only to find the only thing near it, Tagomatic, was somewhat lacking (you couldn't browse, it was full of spam and the recommendations weren't much cop). IMDB has a recommendations feature, but again, not that accurate.
The idea's still valid though, and as they showed, could easily be applied to more items. It seems to be fairly common for recommendations for music, find similar artists, etc, but not so much for books and especially films. Which seems a little tragic. So, while I don't seem to have the time, I thought I would launch the idea out there and see if someone else could take it and run with it.
I think it would need to be community driven, automated systems wouldn't cut it. It would need to be simple to add a recommendation, a text entry, no login (unless you wanted to, then perhaps you could attribute recommendations to people and have a points score or something, might encourage return visits). A page for each film with listed recommendations, with links to those films (some details would be good so you can get at least a plot synopsis, again, possible to allow this to be added by the community).
Each recommendation would have a vote attached, how many agreed, how many disagreed and a remove option (with a some sort of rule that if the votes were negative and you had three people click delete then it got removed). Maybe only allow registered members to delete. Possibly add the extra functionality of comments to allow people to express why they recommended a film to this one.
RSS feeds, maybe notifications and a way to browse by title (you don't want people to have to add more than a title really, which you then need to try and consolidate if people put in an incorrect/slightly different varient), I would say by genre but that seems too much work.
Obviously neat-looking and ajaxified to make it nice and quick. Anyway, not an easy task, but a good site if you can get it running.