If you are running OS X Tiger, iChat is a Jabber client. There’s also the open source OS X instant messenger Adium. Linux users have Kopete and Gaim. Windows users have Miranda, Exodus, Psi, Trillian Pro, and many more.
If you have a Gmail account, you’re good to go. The system integrates your IM buddy list with your Gmail address book, automatically. Login is your gmail user ID (minus the gmail.com bit). Password is, duh, your gmail password. Server is talk.google.com. Protocol is Jabber. Google have detailed instructions available.
“Great,” you’re saying, “Another IM system.” Except that Jabber interconnects with MSN, AIM, ICQ, IRC and Yahoo chat. The servers can gateway the proprietary protocols for you, so you can use a Jabber client to talk to everyone, and don’t necessarily need a multi-protocol IM client.
If you don’t want to use Google’s server, there are many public Jabber servers available. They all interconnect in one big network.
I think this could be the tipping point, the thing that makes open interconnected instant messaging take off. In a couple of years the closed networks might be forgotten, just like nobody now quotes a CompuServe ID or a BITNET or UUCP address for their e-mail address. IM will follow e-mail into an era of open interconnectedness.
Google’s server seems to be having a few scalability problems this morning, which I take as further evidence of my thesis. So get with the program, and switch to Jabber. All the cool kids are doing it.