After being inspired by Girl Talk's impressive mash-ups that compiles 200 songs into a fascinating hodepodge, I decided to try out AudioMulch. It's a modular audio software that can synthesize sound and process live and pre-recorded sound in real time.
Like Reaktor and Max/MSP, it's a patcher-style interface where various modules called contraptions can be connected to route audio and make a wide range of sounds, with up to 24 channels of real-time audio input and output. Unlike its competitors, however, it only costs $89 for the license and has a much simpler interface.