I thought I'd list the web sites that I frequent for finding applications.

AlternativeTo
VersionTracker
Tucows (mostly Windows)
nonags (Windows completely-free-ware)
I Use This (mostly Mac)

I came across AlternativeTo recently, and I really like it. It needs some more users, to hit critical mass, but it does a pretty good job of actually presenting applications to you. For one thing, since you can only vote for, not against, not rate, you don't get the only-complainers rating problem.