With the MIDI 2.0 specification moving forward, more and more new companies are joining the MMA.
EASTWEST (soundsonline.com) is one of the largest virtual (software) instrument companies in the world.
Gewa (Gewamusic.com/) makes digital pianos
Ploytec (Ploytec.com) develops chipsets for USB MIDI and audio as well as the PL2, analog synthesizer module
Second Sound (secondsound.com) makes chips for audio-to-synth technology that have analog synth components and translate audio into CV or MIDI signals to control pitch.
Spectrasonics ( Spectrasonics.net ) makes Omnisphere, Stylus, Trillian and Keyscape- some of the most popular virtual software synths in the market.