Announced not too long ago on his Twitter, Garry is looking for some ideas for achievements in GMod. Suggestions are still being taken over in the unofficial forum thread. Come up with a great idea for an achievement, and you just might end up seeing it added to the game!

Update - 7/30/09: As seen on Garry's Blog, he has done some substantial work on the achievement system that's coming to GMod.

Before - - After

From the look of things, it shouldn't be too long before the release of the achievement system.