ReBirth RB-338 v2
On first seeing Rebirth I was very impressed. It emulates (and looks like) two TB-303s, an TR-808 and a TR-909. According to its creators, they analyzed the circuit diagram of the real machines and coded them into the PC. All controls work pretty much like the real thing, and it sounds indistinguishable from a real TB303 - much nearer than devices such as the Novation BassStation and the Deep Bass 9. It is also possible to use MIDI controllers instead of the mouse, allowing physical control and control of several parameters at once.
There are a few effects units - a delay unit, a distortion unit, a compressor (perfect to run 909 kicks through) and the best new feature - the PCF. The PCF is a pattern based filter which means it is a filter based on an envelope (or pattern as they call it) which triggers every time the current sequence restarts. This can be set to bandpass or lowpass and gives some really great effects, especially on drums. One of the most powerful features is the sequencer, which allows complete songs to be built up independant of a sequencer. You can build up 4 banks of 16 patterns each on each of the four units. Each pattern has up to 16 notes in it. Patterns can then be chained together to form songs.
Other new features are mods, which are new skins (looks) for ReBirth that also come with new drum samples. ReWire lets ReBirth run multichannel ouput directly into a supported sequencer (such as Cubase). Rebirth works with DirectX (for lower latency) or with MME audio. It costs about $199.