If you're referring to games I've started but not finished, I couldn't even begin to count.
As far as games I'm currently playing, that's pretty much constricted to 2 games: Half-Life, and Half-Life 2.
I leave a long trail of unfinished business in the gaming world...
There are also probably a ton more, I just can't think of all of them...
CoD: Black Ops
Halo: Reach
A Link to the Past
Legend of Zelda
Kirby's Adventure
Majora's Mask
Luigi's Mansion
CoD4: Modern Warfare
CoD2
Halo 3...
Most of the Zeldas (refer to my signature)
Halo: Reach
CoD: Modern Warfare & MW2
Goldeneye 007 (N64, not Wii)
Super Mario 64
Metal Gear Solid: The Twin Snakes
Also some more that I probably forgot...