I use Ableton 8 Live, and Massive.
But another great one, if you really want a good interactive-experience and a more real-feeling program, get Reason.
I got my hands on it and I can't stop using it.
Reason is like Massive, it's a side-program you make music/synths/sounds with.
It gives you drumpads, synthesizers, wavemakers, VOX alteration, Gates, etc.
But if you want two sounds to really run together, you can click "Tab" and it'll show you the back of all the synthesizers and you do the wiring and everything yourself.
http://1.bp.blogspot.com/-q8_RSCpdJ5...reason+pic.jpg http://media.soundonsound.com/sos/fe...lcabling.l.jpg