I really enjoyed play testing other people's games. It really opened up my concept on what we actually were able to do within game maker.
The three games I played were: Infinite Boring, Fairy Impact Recoded, and 2012_Alien-Apocolypse. And I felt that these games were rather complete enough to test, yet still had some interesting problems. While some of the mechanics were relatively the same (you the player moving a sprite representing yourself through space) mechanic of input for the game.