Google Code has published (created? sponsored? I'm not sure how that works) a plugin for the Pidgin IM aggregator client that works with Facebook chat. Once it's installed, you can add your Facebook account as a new account, and it works just like every other chat network.
This is great as far as I'm concerned. I use Pidgin and am not a fan of Facebook chat (or web-based chat in general). Some of my friends use Facebook but not regular IM.