oubliettezombie demonstrates how to turn your thumb into a pad synth sound:
In this video, I take the sound of my thumb moving around on the microphone (Shure SM57) and turn it into an atmospheric pad sound.
The most important step in this process was scanning through the recorded sound and setting the loop points in such a way that the source sound became more interesting. After that, things like filters, envelopes, FM, etc. can be applied to really give it life.
All manner of found sounds contain individual waveforms that are far more exotic and complex than the basic waves found in synthesizers. Sometimes this is desirable, sometimes it is not.
If you know of other interesting examples of sound design, leave a comment!