Francis Preve explains how to use Ableton’s Simpler to create your own wavetables in this article.
WaveEdit is the free, open-source wavetable editor developed by Synthesis Technology. You also can just download Wavetables for free.
This free M4L device made by Dillon Bastan and Carlo Cattano let´s you create custom Wavetables for Ableton´s Wavetable synth from image files. Get it from www.maxforlive.com
Cool Reaktor wavetable synth by Michael O’hagan. With this one he became one of the winners of the recent builders competion by Native Instruments. Includes 5000 waveforms and multiple modulation options. Download from the Reaktor User Library.