I use effectrix by sugarbyte and livecut on mac OSX, the last one is free but produces things with a kind of chaos parameter so you can't replicate the same effect each time. You have to record and select what is right for your song, but it works very well, and you can't do the same things with effectrix.
All in all I tend to use more effectrix but it depend what I'm after. Effectrix seems to be a bit more warm than livecut.