It has to be either Half-Life 2 or Mirror's Edge. When I just don't feel like playing anything in particular, I can always start either of those up and just loose a couple of hours no problem. Recently I started playing Mount & Blade Warband again, so maybe that'll be one as well. I also do dabble in Kerbal Space Program a bit, but I think I might be one of the worst players of that game ever. My usual solution to any problem is to strap more solid fuel boosters on my rocket. The results tend to get... messy...