Single-Player version is possible, but you'd need to get a ported version to run it off your own computer. The IGZ was bought up by Microsoft and now is run for free by the company. The IGZ's a great place to play games like bridge, chess, spades, hearts, othello, and so on, but it's not big on community. The network just wasn't built for communication. Now IGZ has games like Jedi Knight and Age of Empires.
Some games require retail version.
Trial versions of those games are available. Multi-Player Web Site Ultima Online at the moment looks like the best graphical game available for multiplayer. Diablo sucks in comparison. No massive hacks or bugs, either. Ultima Online easily outdoes The Realm and other online games.