Seconding Ableton. Live combined with the Push 2 is the best computer interface I've seen for anything. I've often found myself starting a project at 9pm at night and just getting lost tinkering with the sounds. The next thing you know it's 2am and I'll have a headache at work tomorrow.

You can do the same thing with a keyboard and mouse, but the Push makes playful experimentation so much easier, tactile, and fun.