A: 

Generally, you figure out something you want to do, then search for a gem, not browse through gems on the off chance you hit functionality you find useful.

That said, you could search github: http://github.com/search?type=Repositories&q=gem

But the best idea would probably be to go look through other open source projects and see what they're using.

Jamie Wong
@Jamie, when I look for extensions of firefox and chrome, I found they have a really good web site. It's easy for a beginner to find good extensions they want. Hope there is a website like them for gems.
Freewind
+3  A: 

The Ruby Toolbox is quite useful to find gems. They are sorted in categories and ranked by the amount of watchers/forks on github. No voting/discussions, though.

dhofstet