Ableton Certified Trainer Yehezkel Raz presents another unique download - this time, it's a set of two Max for Live devices. Peaks responds to incoming audio transients, and can be mapped to any parameter in Live. Notes, meanwhile, translates velocity data from its main knob into MIDI note messages. Used together, you can trigger a MIDI instrument with audio input - see this demo video for an example:

Get the devices (along with many others from Yehezkel) at 4live.me