If you're using Rails:
- searchlogic: incredibly easy searching of ActiveRecord models. I've cut out huge amounts of code by using this gem.
- factory_girl: get rid of those awful fixtures!<p>Also, even though it's not a gem, JRuby is an awesome platform - I actually prefer it to MRI after using it. It's incredibly stable, fast, and bugs are fixed damn quick. I reported a bug in JRuby-OpenSSL yesterday at about 9am, and there was a fix done by 1pm. Very impressive.