Hmm...if you want to play with other people, and are looking for a quick (or maybe not so quick

) waste of time, League of Legends is free to play and might be fun. I've never played it personally, but my friends are completely addicted to it so there must be SOME merit to it. OR, you could give into the Blizzard mongrel and start playing Starcraft, if you want some hardcore real-time strategy games up in here.
Can't think of anything else right now except for this open source game Battle for Wesnoth, which is simple, but strangely addicting...