Update: Reaktor is now at version 5. It's just got better and better, and while it now has some strong competition (especially MAX/MSP) it's an incredible product with a fantastic user library. It's easier for me to say what's missing rather than what it has, as there's not much that isn't here. There's no ability to host child VST plugins, and not FFT. But for pretty much anything else it's a true swiss army knife of software synthesizers.
I have only one word to say.....wow! Generator/Reaktor is the most amazing soft synth I have seen yet. It is a modular system where you insert modules on the screen then draw lines between them to connect the modules' various inputs and outputs. This is the structure of the synthesizer. You can then go into another window where any knobs, sliders or LEDs you have created appear on a graphical panel. It is possible to map any of these to a MIDI controller just by selecting the knob, pressing the MIDI learn button and twiddling the controller on your keyboard. Snapshots can easily be created of panel settings which can then be recalled using MIDI program changes.
It is impossible for me to state the flexibility of this program. It can do anything from simulating a sampler to a vocoder to a near-perfect emulation of the Roland Juno-6 (I did a side-by-side test and was impressed). Many macros are provided, which are collections of modules bundled together. Examples are oscilloscopes, arpeggiators, TR-808 type step sequencers and distortion units. This makes it very easy to piece together a custom synthesizer. An update soon to be released will also allow you to use VST and DirectX plugins as modules.
Reaktor is a superset of Generator - it has all of Generator's features plus extra sampling, formant and sample resynthesis functions from Transformator. It can be linked to a sequencer for a very powerful setup - with either the sequencer or Generator/Reaktor providing the master clock.
Generator can be used with a normal soundcard, with 15-300ms lag, depending on the card and its drivers. It also has native support for the E-magic Audiowerk8 card with 15ms lag and 8 individual outputs.
Reaktor 5 costs $449.