Controllers

• To hear the sounds from a virtual keyboard (or any software instrument), a hardware controller is needed. Controllers have no sound and are usually keyboards cut down in size like the M-Audio Oxygen.
  
  

Virtual Organs

• Virtual organs are normally copies of a hardware counterpart, like Native Instrument's B4, an emulation of the Hammond B3. Other companies that create virtual organs include East West, Linplug, and FXpansion.
  
  

Software Synthesizers

• Virtual soft synths are categorized by the way they create sound. Some of them include FM synthesis, analog, subtractive, additive, wavetable, and modular.
  
  

Virtual Pianos

• The most ambitious endeavor to emulate an instrument are virtual pianos. Companies like EastWest, Synthology, and Steinberg have released piano versions of the Steinway Concert D9 and the Bosendorfer 290 9'.
  
  

Software Accordions

• Though technically not full keyboards, accordions do have keyboards to operate them and there are (believe it or not) software counterparts for that also.