Tools:

Engines:
Game Maker wiki available here
Python w/ Pygame
Unity game engine

Angel created for EA game jams
Scirra
Love 2D game engine
XNA with Visual Studio C#

UNreal

Environments:
Scratch http://scratch.mit.edu/
Greenfoot http://www.greenfoot.org/about/

Art:
Spritelib
Sprite Walk Cycle tutorial

http://www.manningkrull.com/pixel_art/tutorials/walking.asp
GIMP
Sumo Paint
Inkscape
Lost Garden Graphics
Frogames [Dungeon Pack]
Paint.net
Google Sketchup
Wings 3D
Blender


Audio:

Audacity
SFXR: randomly generates sound effects

Hyperscore