I enjoy agent-based simulations; I first got into programming as a way to explore mathematics and visualizations.
I'm also really interested in public policy and public data. So I like playing with statistics (especially R) and visualization too.
Lately (2011), I've been doing most of my programming in Ruby and Scala. I also enjoy Python and Haskell. I've spent some time learning Clojure and probably should find a project to do with it.