With lack of anything better to post, I'm going to post a list of old games that still work on my crappy desktop. These games are still so goddamn fun, and people mod them continuously, so the fun remains forever it seems.
Grand Theft Auto is still amazingly fun. After I got about 75% through the game I discovered the MultiPlayer mod, and that keeps me entertained until I take the time to finish the game. The multiplayer addon can be found here: http://sa-mp.com/
WarCraft is a great game, but I'll be the first to admit I've only played through the first 4 levels. I've had random battles online when there are no custom games to play, but my biggest time-waster is DoTA. If you haven't heard of it, it's basically like a hero team vs another hero team. Their are troops, but they move in a set path. You win if you destory an enemies base. It has a large player base and several large scale competiitons. The pro players earn 10's of thousands of dollars as they win competitions. If you know the torrent game and would like to use it without paying, you can't play on Battle.Net.But there are several alternatives.
Garena: Garena allows you to play even if it's not official as in your serial key allowed it to pass the setup, but not actually play.
Download the DoTA map: http://www.getdota.com/
Trailer for the latest DoTA (Gonna be AWESOME): http://www.youtube.com/watch?v=LNTT5J54V7w
In conclusion, Google is your best friend. If you need some help, some guides and etc just Google it. Or leave a comment or an email, and I'll get back to you.