Many great guilds out in these waters, quite a few will have at least a small group of their members using an external chatting program (Xfire, as Del said, is a big one. I think a few others will use Steam.) Anyway, there are a load of guilds that are on these forums and can all be found in the
Guilds and Clans forum, along with the three active Subforum Guilds:
Black Bart's Krewe,
Partners N Crime, and my own
Shadow Sorcerors. Best of luck finding a guild!