JN Soundboard - An open-source hotkey soundboard program
CasterSoundboard - A soundboard for hot-keying and playing back sounds. (For podcasting)
Soundplant - Play sound files with the computer keyboard
Soundboard Studio - Soundboard Studio lets you take your podcast, live show, gig or performance to the next level.
Jingle Palette - Jingle Palette application was designed to be an instant jingle player for radio broadcast studios.
Farrago - Intuitive soundboard app to quickly play sound bites, audio effects, and music clips.