Well,
Reason is an industry standard, but it is probably way too complicated for what you want to do, and costs way too much money, but you can make pretty much any sound you can imagine, and a whole bunch that you can't.
I use
Mixcraft for recording, and have been for several months with no problems. I don't do much MIDI stuff, but it is a lot simpler to use and not near the resource hog Ableton is, and you can get a free trial. You should be able to use it as a MIDI controller without recording if you want.
Does anyone know if Audacity has MIDI support? I haven't messed with it too much. Or any other decent free MIDI modules?