Ok, it's not the most advanced tool, but it's pretty nice.
Basically, you edit the playing field, add enemies, power-ups,
walls, etc.

You can make a game and publish it! This tool is a beta btw.


My Game: Super Awesome Space Arena!

You and your 5-Man Squadron must eliminate all of the
enemies and save the galaxy! You'll be pitted up against Guards,
Speeders, Cruisers, and the most deadly of enemies! TURRETS!
WARNING: VERY LAGGY CUZ OF ALL THE ENEMIES. AND REALLY HARD

http://www.sploder.com/publish.php?s=ce0e5qu4&browse=1&ref=0

EDIT: I just made a new game. Base Infiltrator, it's pretty tough.
My username is "Duelpro".