Mostly 3.5, a bit of other d20 stuff. I'm about to DM a Pathfinder game, and I've played in a handful of others (all short) over the past year or two. Currently playing in one 3.5 game and a 3.5 PbP, DMing two 3.5 games, and now playing in an M&M PbP. Also DMing a d20 Modern/Future game, but it's been erratic with lots of player absences and a few times where I had to cancel.
Still trying to find someone to play Chronicles of Ramlar with. Or Arcana Unearthed/Evolved. Or EoM: Revised, or EverQuest d20, or Shadowrun 3E (though at least I've played and GMed a handful of SR3 sessions before), or various others...... And still hoping to find a new group for 3.0 D&D. At some point I'll resume playing/DMing 2E AD&D as well, just not sure when I'll get around to that (don't have my Core Rules 2 CD-ROMs installed).