Kapling
Kapling is a simple little standalone physical modeling synthesizer application for Mac OS X built with Max/MSP 5. It is based on the Karplus-Strong algorithm. Kapling uses this method to attempt the emulation of resonant bodies, like a string being plucked, or melodic percussion instrument being struck. Eight independent voices are triggered by a 32-step sequencer, with a variety of parameters set up to help you sculpt new timbres in realtime. Kapling also features a tempo-synced stereo delay and recording to disk. In simpler terms, Kapling can help you make twangy, boingy, klonky tones. And it's free. (Donations welcome).