I always like a synthesizer that takes a different approach. Most of the software reviewed on this site emulate analogue synthesizers and use subtractive synthesis. Chaosynth is different - it uses a combination of cellular automata and granule synthesis. According to NYR Sound, who manufacture Chaosynth:
These consist of a group of cells that evolve over time following a pre-defined rule. At each time segment, every cell changes its state depending on the conditions of its neighbours. Starting with a random state, the cellular automata quickly settle to form organic and eventually oscillating patterns. When this is translated into sound, a complex evolving waveform is formed with a rich texture.
This gives you an idea of the sort of sounds Chaosynth produces. It contains a lot of flexibility - a lot more options than I have seen in a granular synthesis program before, and it is very good at its job, producing some very interesting and different sounds. It will be very interesting to see where these techniques go in the future. A demo version is downloadable from the Chaosynth website, and the full version costs around $40.