Firstly, I'm a fan of Valve's games. Half-Life had me hooked from day one, the expansions allowed us to view the story through parallel POVs, and its mod-friendly format helped Counter-Strike blossom into a multi-player powerhouse. HL2 surpassed its predecessor in nearly every way, and following episodes have continued to raise the bar. By hiring community developers to produce Portal and Team Fortress 2, they've expanded their repertoire into the puzzle and team-based genres. Left4Dead is high quality co-op multi, though best played with "real" friends and voice chat.

When they first introduced (and required) Steam, I hated it. Over time, however, it's grown into a worthwhile online platform. Whether by its vast improvements or Stockholm Syndrome, it's won me over. (definitely compared to Games for Windows Live... yuck) L4D is my main multi-player game right now, but I also play the occasional game of TF2.

Anyone interested in adding friend info, etc? My handle is 'deadly.by.design' for those who are interested.

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Multi-player games available: (whether I play them or not)
Counter-Strike Source
HL2 DM
Left4Dead (most played, currently)
Team Fortress 2
Unreal Tournament 2007, er... 3