The Covert Operators now have their own Website, and they’re filling it up with tons of sound content for Ableton Live. You may recall the excellent free Covert Operators preset library (most of them for Ableton’s Operator synth); it’s back and newly-updated, for a total of 700 nicely-organized presets. On top of that is the new Sound Wrecks library, with 3000 LiveClips, hundreds of FX chains, and 66 drum kits.
Propellerhead Reason has long had great add-on content (see CDM’s recent update on that), but finally Ableton Live is coming into its own. Add this to the vastly-expanded presets Ableton includes in version 5, plus great soundware from Trackteam Audio, and Live 5 has a really robust set of sounds, on the cheap, no less. Now go mangle them and make them your own.