The Ruby Toolbox – a collection of good gems

If you need a gem for a certain purpose, be sure to check this site.

The rankings are determined by counting up the number of forks and watchers of various github projects, so I'd view it less as "this is what I should be using," and more as "these are some things I should check out." At the very least, they're all likely to be under active development and fairly up to date, and it's very useful to see groups of gems broken down by category.