Synth Site |  Master Zap | Integral Software | Out of Phase

Basically allows you to define instruments, based on either a sample (imorts aiff, wav, and a user-definable raw for importing Akai, Kurzweil, Emu, and so forth) or a algortihmic wavetable. If you're a programmer you can pick up on the langauge pretty easily (I wrote a module to emulate an 8-vco analog synth with pulse-width modulation, phase distortion, and other goodies in less than an hour). Instuments are multi-sample-able, multi-oscillator bits of code with multi-tap envelopes for volume, and filters. Supports something like 9 types of filter (lowpass, highpass, bandpass, reslowpass, reshighpass, bandreject, butterworthbandpass, and a few others). PLus, and this is amazing, a variable-band vocoder (I routinely vocode samples with 128 bands). Basically, you can just keep adding oscillators and parameters and effects since it's all virtual. More you add, the slower the output is generated, but with a few LFOs and some good filters even a single osc can produce some fat sounds.

The sequencer is notation-based, with a pallate for editing various effect parameters for each note. Has SMF import (not output, yet). Section effects for adding reverb, filter, delay, etc to multiple tracks, score effect for adding to the entire thing. Outputs to an AIFF file (I usually dump the finished product straight to CD).

Documentation is only current to 1.3...2.0 is technically still in beta. Demo files accompaning v1.3 are useful and contain some pretty good algorithms (a few analog routines, FM, phase distortion, sample filtering, ring mod, fourier synthesis, basic additive, and a few filtering examples).

And it's free, (Gnu Public License, so it includes source code). Not for the meek, since there's programming involved, but if you dive in it's capable of making some amazing sounds.