One of my favourite sound generation tools (and one I used exclusively for Unstable) is ChipTone. Developed by Tom Vian, it is a tool that allows you to generate retro-ish sound effects very easily, and with a surprising amount of flexibility, that can then be exported in .wav format.
Although you can simply auto-generate simple sound effects such as coin collection sounds or explosions, the real advantage of ChipTone is the way you can alter the waveform by adding different effects (tremolo, arpeggio, wah-wah, etc.) While the tool is supposedly in development, it is more than functional enough to create a whole library of sounds that – and this is significant – have a more consistent feel to them than you would find by importing various free sound effects from different sources. Combine the results with a sound-editing tool such as Audacity if necessary, and you have all the power you need.
I won’t turn this post into a tutorial of how to use ChipTone, since the best way to learn is just by playing around with it. (In fact I’ve had great fun just clicking the Randomize button and then tweaking the results, an activity I can seemingly waste hours on). So you should check it out.
Did I mention it’s free?