Ask YC: Great software companies to work for in DC?

I'm looking at moving to DC at some point soon, and I would really love to work for a great, medium-small software company. Based on previous experience, I feel I'd be much happier at a smaller company (being at a "big corporation" hasn't been so exciting for me) that focuses specifically on developing software, rather than a company where software is in support of other developments. The list may include, but by no means be limited to, software startups. Any ideas?