bfxr - An online chiptune sound generator.
LabChirp - Create simple sound effects on Windows.
SFXR - SFXR is a program that specializes in making chiptune sound effects.
ChipTone - Chiptune sound effect generator.
sfxia - Free retro style sound generator for games.
rFXGen - Simple and easy-to-use FX sounds generator inspired by sfxr.