boom lets you access text snippets over your command line. I'm personally aiming for exactly two use cases, but I'm almost positive there are thirteen more. Here's a couple examples:

Your own del.icio.us-esque URL tracker. When I make clever animated gifs of my coworkers, I tend to lose the URL, which is a total bummer since I want to repeatedly repost these images well past their funny expiration date. boom lets me easily access the good stuff for years to come.
Commonly-used email replies. Everyone's got those stock replies in their pocket for a few common use cases. Rather than keep some files strewn about with the responses, boom gives me them on my ever-present command line.
Simple todos. You can super-quickly drop items into lists and remove them when finished. I'm a big fan of simple, straightforward stuff. Plus, it's a Dropbox away from simple cloud syncing. Someone get Cultured Code on the line THIS MAY BE RELEVANT TO THEIR INTERESTS!
We store everything in one JSON file in your home directory: ~/.boom. The structure is simple, too. Each individual item is tossed on a list, and you can have multiple lists.

rush is a replacement for the unix shell (bash, zsh, etc) which uses pure Ruby syntax. Grep through files, find and kill processes, copy files - everything you do in the shell, now in Ruby.

calllog2ical

BlueCloth is a Ruby implementation of Markdown, a text-to-HTML conversion tool for web writers. Markdown allows you to write using an easy-to-read, easy-to-write plain text format, then convert it to structurally valid XHTML (or HTML).