BlockSite

I use the Firefox add-on BlockSite at work. I found myself spending a lot of time on "goof-off" sites (digg, facebook, linkedin, wikipedia, youtube, phdcomics, etc). While I wouldn't stay at any particular one for very long and each of those sites is useful to some degree, I think my productivity increases by just blocking them. Since I use Firefox I just installed the BlockSite extension and off I went.