Well, I'm still in the process of looking for one myself.
So far, I've found the Crystal synth, which can be found here.
And I've also found ProComposer.
To mw (who has tried out neither of those two), Crystal looks like is was made for the professional, while ProComposer was made to look like your regular keyboard-synthesizer-on-the-desktop.
And besides synths, if you want to try, I also recommend VirtualDJ (if you can afford it). You can play just using the mouse, but of course, it would be easier if you had a turntable you can hook up to the computer.
