Polyrhythmus is a Max for Live rhythm sequencer, arpeggio builder as well as a MIDI CC and parameter automator. If you're into polyrhythms or arpeggios, this one's quite a treat. It looks great, too and despite all this is free.
I personally prefer real analogue synths, but they're often not exactly cheap. If you're looking for a modular synth that integrates seamlessly in Live and you have Max for Live, Dope Matrix may just be the thing for you. It's