Facebook is exploding these days, there are tons of applications that can be installed within profiles, but there is a new application on the block that installs on your computer instead of in your Facebook profile.
Social.im is, according to their site, an IM program that let's you chat with all of your Facebook friends:
"Your friends are spread across a bunch of IM networks:
Yahoo, MSN, AOL, Google, etc.
but they are all on Facebook!
Chat with all your Facebook friends from one IM client."
And the best part about Social.im? It runs as an an Adium or iChat plugin, so no need to install 3rd party chat software!
Social.im is freeware and can be downloaded here.