Though it's a sad comment on humanity, apps that play sound effects (preferably crude ones) are a popular source of entertainment. It's in our DNA. I'll bet that early cavemen invented the whoopee cushion before they mastered fire.
But if your sound effect needs are beyond the scope of your garden variety flatulence app, there's an excellent and free piece of software out there called SoundPlant that allows you transform any Mac or PC into a custom soundboard.
Honestly, the idea for this How To came to me as I was walking through my local Halloween store, noticing how everything seemed to have a button of some kind that would make some creepy sound. I couldn't bring myself to spend the extra on the auto-howling wolf zombies because I kept thinking about how much better, scarier, louder, and cheaper things would be if made my own sound effects system using my computer linked up to some cheap speakers in the front yard.
Whatever this inspires you to do, share it in the comments section.