Wow there are a lot of great games on this thread! I only have one to add, and that is Exodus3000. It's a free multiplayer RPG strategy game where you can have fun while earning real cash and prizes. Be warned though, it's addictive. If that sounds like you the link is http://www.exodus3000.com/?r=dreadpirate.
Most of the games I'd recommend are already listed here. But one that's missing is an online game called Exodus3000. I've only been playing it for a bit more than a month, but I'm addicted, lol. You can find it at [url="http://www.exodus3000.com/?r=dreadpirate;]. Enjoy!