For those of you with Steam and Half-Life 2, check out Codename: Gordon. It's a 2-D cartoon sidescroller game that goes through the entirety of Half-Life 2 in like 7 or 8 boards. Absolutely hysterical. The commentary from the characters you meet is also brilliant, with just about everyone bitching about losing "that third dimension" and so on. I was laughing my head off last night during "the boss fight" with the gunship. You will get spoiled if you haven't already played through HL2 to completion, so be warned. But for those who have, this is a brilliantly fun and simple freeware download.
Just thought I'd pass the word along.