bfxr - An online chiptune sound generator.
LabChirp - Create simple sound effects on Windows.
sfxia - Free retro style sound generator for games.
rFXGen - Simple and easy-to-use FX sounds generator inspired by sfxr.
Audio Hero - Over 250,000 royalty free music & sound effects for videos
Power Station Industrializer - Industrializer is a program for generating percussion sounds for musical purposes.