AIR, formerly known as Wizoo, acquired by Avid a few years ago to develop plug-ins for Pro Tools, and recently dropped by the aforementioned Avid. They've created a bunch of very nice instruments for Pro Tools.
Sonivox, who entered the Virtual Instrument market by re-packaging old Sonic Implants Soundfonts in a shinny GUI... Their new instruments may be a bit better, though.
Apparently they are teaming up to develop instruments.