After reading Mark Chu-Carroll's extremely interesting article on using lattices for representing information, I started a hunt for good graph representation and analysis libraries. I looked for Lisp, Ruby, and Java libraries, and found a great looking library written in Ruby (gratr). One caveat: I spent 30 minutes enjoying reading through the library source code, but otherwise I have just experimented with the tests and examples. Thanks to Shawn Patrick Garbett, Luke Kanies and Horst Duchene.
I often find interesting/useful software this way: I get excited reading a good technical paper or blog, and then go and look for relevant software tools.