Orangator
Orangator is a very powerful synthesizer. It features 10 oscillators, each of which can be sine, square, saw, triangle, noise, a sample or a custom waveform (you draw a cycle on the screen). There is also ring modulation, sync, phase modulation and frequency modulation and you can directly alter the harmonics produced by individual oscillators. There are also 3 envelopes plus a custom (draw-it-yourself) envelope, two LFOs, multiple filters (including high/low pass), a 32-note sequencer, reverb, flanger, chorus and overdrive.
However I didn't have much luck with the realtime mode - it doesn't seem to support very complex synths before it runs out of processor power. This is because it was designed as a non-realtime program first and then had the realtime mode added on. There is also a modular version available called Visual Orangator, but this does not support realtime.
