I use reason 2.5 (
http://www.propellerheads.se/), very powerfull program. Also tried Native Instruments Reaktor 4.0 (
http://www.nativeinstruments.de/index.php?home_us) just some days ago ... seemed kickass but hard to learn. Its like reason in some senses .. you can connect devices in long chains to get the effects you want.
Started on a housetune some days ago with reason, actually with a sample posted here on GFY by fletch I think around a week ago :]
Short clip since I just started:
http://207.44.152.26/on_dope.ogg ..
For you who doesn't know ogg, its a license free format with better quality then mp3 .. (
www.vorbis.com). Newest 2.x and 3.x winamp versions support it.