Bfxr is a sound effect editor, presenting itself as a successor of Sfxr with added complexity and range of options. You don't need to be an audio expert to quickly understand the diversity of outputs one can get from this tool just by clicking the "Randomize" button.
Best of all it comes for free and online (with a downloadable standalone AIR application, if you need it)!
What's your experience with Bfxr? Do you know of any other similar tools?