(edit: Construct 2 is going Closed Source, I'll either edit out this part or keep it for reference, but don't get your hopes up when reading this)
Scirra Construct is one of the few FOSS competitors to the proprietary GameMaker 2D game development suite (Gluon, Enigma and Game-Editor being the others). Unfortunately, the system has been limited to Windows since its inception due to relying on DirectX fiddlies, however, that has all changed...
Construct 0.x Showreel
Yesterday the long awaited Construct 2 project released a public alpha. No longer are Construct games chained to Windows; instead, games can be exported to HTML (5) which is pretty much on the opposite spectrum of platform-lockin.
|Construct 0.x Interface|
Jump on their forums if you're interested!
Why HTML (5)?
"Game On" competition where developers entered games that utilize open web tech; and FOSS web games won 2 of the categories ("Best Webbiness" and "Most Fun")!
Don't write HTML 5 games off to 2 dimensions either, someone recently posted a rather simple but fun 3D RPG game utilizing WebGL on OpenGameArt the other day, it's rather cool :) Also don't forget about the Quake 2 WebGL port! (remember, these require a WebGL enabled browser, such as Chrome/ium and Firefox 4)