K-Devices has released the Rad Envelope Follower (REF), a Max For Live envelope follower with advanced features.
REF is a great tool for any kind of modulation: kick/bass side-chain combinations, driving filters & experimental uses asking for controlling parameters from an audio source.
It allows you to modulate anything you want in Ableton Live: other devices, the user interface, and third party plug-ins, using sound as source.
You can shape the incoming audio source with the low/high pass filter, listening the source in peak or rms modes, then design the envelope exactly how you need it, using tools as relative/absolute modes, linear/logarithmic modulation, and positive/negative directions. You can even work on your envelopes with standard EG parameters as attack, hold, release, and a global delay time.
REF for Max For Live is available as a free download until May 31st, 2013 (regular 3.99 EUR).