Really, it all depends on what you are doing :).<p>For example, Geokit rocks if you need to do, or add any geoencoding stuff.<p>If you can get past the hell on earth that is installing ImageMagick, then RMagick is amazing for image processing and handling.<p>If you need to write HTML, Haml/Sass is your new best friend, if you need to parse it, Nokigiri or Hpricot both rock.<p>If you need to track states in your objects, statemachine or eventmachine is pretty useful, and +1 to the commenter who pointed out ThinkingSphinx, that's blazingly fast for full text indexes.<p>The daemons gem is really useful if you want to daemonize a process, and create quick scripts to start background applications.<p>workling/starling are really good for background work as well, as is delayed_job.<p>It also depends on what ruby version you are using. I'd recommend 1.9, as we've managed to get patches for pretty much everything we run, or hacked the gems themselves to work, and it <i>is</i> faster than 1.8.6. That said, you do still run into problems.<p>There's loads more. There's more testing frameworks and unit test things than you can shake a stick at.<p>As I said it all depends what you are using, and what you are using it for.<p>Hope my ramble was informative! :-)