Although I'm sure that I completed games before it, the first game that I explicitly remember getting 100% on was Metroid Prime. I have probably played through that game in its entirety at least a half dozen times, not counting partial playthroughs. I also got 100% on MP2 & MP3. Also, back in college, my roommates and I played countless hours of Smash Bros Melee and got 100% of all the unlockables in that game (characters, stages, music, trophies, etc).
As far as more current games go, I completed 100% of both Assassin's Creed games (yes, even the pointless flags in the 1st game). I got 100% on the Complete Saga version of Lego Star Wars. I've got 100% on Bully. Technically I've played through 100% of Oblivion plus expansions and Fallout 3 plus expansions, but those games are so large I'm sure there are a tons of nooks and crannies that I never found. At one point, I had done 100% of Dragon Age, but I haven't played any of the post-launch DLC yet. Most recently, I got to the 100% mark in Red Dead Redemption.
I tend to try and get the most out of my games, so I at least like to attempt to get to 100% (especially if the game tracks that sort of thing).