Wavepeek is a free M4L synthesizer with a wave-shaping section that is meant to be easy to understand but very capable of creating interesting waveforms at the same time. It gives you full control over two sets of waveforms and morph between them using an EG, LFO, or manually. It works best when you use the shape as a filter, so make the bottom waveforms smoother and lower but you can do some other crazy stuff with it as well.