I reasonably like that I have far more Facebook friends than I have on most My IM clients. So it is very handy that I can use Pidgin to keep up on who is logged in to face book with out actually having to leave Facebook.com open all of the time.
The Facebook chat plugin for Pidgin (here) and libpurple messengers. It connects to the new Facebook Chat IM service without the need for an API key.
Currently the plugin can log into the Facebook servers, grab the buddy list, send/receive messages, add/remove friends, receive notifications, search for Facebook friends and set your Facebook status.
Pidgin also has support for Myspace which fewer and fewer of my friends are on, but is still handy for catching them when I’m not in Myspace (aka never).