Uhm... Alot... Let's see...
I fire up Killing Floor, Left 4 Dead 2 & Team Fortress 2, once or twice a month...
I've been trying to finish off the Half Life 1 expansions for about 3mo now...
Haven't started on Monkey Island 2 after finishing first a few weeks ago..
Been trying to play through Plants vs. Zombies, but kind of meh since I already played through it once year ago..
I still haven't finished Portal...
Got 3/4 way through SiN but got semi bored & uninstalled about 2 years ago, haven't gotten around to fplaying through all of it again & finishing it...
Been trying to knock off these Zombie Driver achievements, but burnt out on that as well...
Been working on Fallout 1 since last year, but meh on that too...
Biggest thing holding me back on majority of those, is not being able to play for extended periods of time without taking a toll on my wrist which is already uncomfortable from the carpal tunnel. PC gaming could use alot more games that thoroughly utilize a controller. Most that should don't, or severely half ass it.
etc etc etc - qqq