The MUDdex: Multi-User Communication Options

Sometimes you need more than a game's built-in chat facilities to talk with your fellow-players.

Voice Communication Software

Why would you want voice communication software if you were playing a multi-player game? Well, truth of the matter is that most games have clunky chatting interfaces. If you want to actually get something done, you have to do it "out of band." This is especially true for real-time combat. Typing instructions to other people is slow. Voice is fast.

Also, if people can talk to each other, it is no longer true that on the internet, no one knows you are a dog.

The two most-used voice communication solutions are:

Teamspeak
and Ventrillo.

These programs have servers for Windows and Linux, but clients only for Windows. Both of them promise Mac clients "real soon now." Both sites provide links to companies that will host chat servers for you.

BBS Systems

If you want persistant messaging to communicate with your Guild or Clan, you probably want a BBS of some sort. Sometimes BBS's are and end of themselves.

Citadel BBS has a long and interesting history.
The web board system of choice for many gamers is phpBB. If you are running pbpBB, be sure you have installed the latest security update. Includes links to cheap web hosting services.

Multi-player Game Hookup Sites

Many games that are more usually played against computer opponents will allow you to play against remote opponents over the internet. Services exist to help you find players.

Game Ranger is a service for Macintosh players.
Game Zone will connect PC and console game players looking for same.
There is at least one more PC game service, but I cannot come up with the name right now. Maybe a friendly reader will help me.

And finally, there is...

IRC. It never went away, and it's found new use for back-up communications for MMORPGs. And if all you want to do is text chat, there is no better tool out there.