We are currently using IRC to hold chats for members, including occasional trivia contests and movie riff parties.
To participate, download an IRC client (for the Mac, I like Colloquy, and I'm told mIRC for Windows is good) and point it to:
server: irc.servercentral.net
room: #bmmb
no passwords required.
If you'd prefer a web interface, I hear there's a plugin for Firefox called Chatzilla, or you can simply point your browser to:
http://chat.efnet.org/
Use the same server info to login, then type:
/join #bmmb
Please note that there is a space between /join and #bmmb in the above command.
Should the IRC server not be working, there is an emergency backup chat room, courtesy of icrywolf.