Internet Relay Chat (IRC) is one of the oldest instant messaging systems. It's focused around topics rather than people, and requires no registration. I hang out much of the time on several channels, including:

 

You can click on any of those links to join the chat via the web.

 

They're all on the server irc.freenode.net. There's also downloadable IRC software for different systems. Some suggestions:

If you want more help figuring out how to use IRC, check out some documentation yourself: http://www.irchelp.org/irchelp/