The Yamaha MU128's four audio inputs let me mix the output of two computers to stereo and capture it to a portable digital recorder.
Got a synth with multiple audio inputs? It can often work as a utility mixer. For example, I wanted to listen to an Electro-Harmonix Voice Box (see the review on page 50) on headphones, but it has a mono output. So I connected it to a line input on my Yamaha MU128 sound module, panned the sound to center and even applied some stereo reverb to it.
In a recent jam, I wanted to play both my Korg Kaossilator and Novation BassStation. I always route the BassStation through a Korg Pandora effects box, but I had never used the Pandora's stereo aux input, which is designed to let you jam along with a CD. Plugging in the Kaossilator instead created an instant performance setup. The hat trick was patching my buddy's and my stereo signals directly to my 5.1 home theater system. He played through the front speakers while I played through the surrounds.