Here is the list of free DAW plug-ins I have discovered so far. I will keep updating this list as I find more interesting plug-ins.
Note, if you're on a Mac, you're encouraged to use the newer Audio Unit (AU) format, if available, rather than the much older VST one. Otherwise, VST should work just fine.
For the AU plugins, copy the .component files under ~/Library/Audio/Plug-Ins/Components folder on Mac and RESTART Ableton Live.
Blue Cat Audio (http://www.bluecataud...)
Ohm Force (http://www.ohmforce.c...)
Audio Damage (http://www.audiodamag...)
Check out their Rough Rider compression plugin. I've mostly used the Live's built-in compression. This unit however is useful for mastering of a completed song, or a recording of a live performance. Just slap it onto the master track in Live and twist the knobs to hear the difference.
Also if you're not familiar with the concept of compression, it's highly recommended you learn about it. Search YouTube for some videos on this topic.
Togu Audio Line (http://kunz.corrupt.c...)
Oscilloscope for Ableton Live - a missing feature. Wanna see what that waveform looks like?
Excellent when designing sounds in Operator and Analog.
Paid Plug-Ins (most have fully working demo versions)
Guitar effects rack. Sounds great out of the box, comes with great presets, easy to understand user interface.
Vocal pitch correction and effect processor.