Having acquired Diablo 2 a week ago, my interest in multiplayer gaming has been rekindled. The nice thing about Diablo 2 is that with Battle.net finding people to play with is a breeze.
Most games lack that particular feature so finding people is a bit more work.
You need to log on to a service like Microsoft's Gaming Zone or use a separate program like Kali or the just released GameSpy Arcade. Kali might be the best known, but it faces some serious competition from GameSpy Arcade. Using GameSpy Arcade should be old hat to anyone who does a lot of online multiplayer gaming. These look and act like a standard IRC chat but instead of swapping porn pics and debating the merits of Rap versus Heavy Metal music, you simply hunt around for other people interested in playing the same game as you. There will be a lot of general chatting going on but because most people are mainly there for the games, you can generally hook up with someone relatively quickly.