I purchased one of these from Johns Organ Works and have been very pleased with it. I use Hauptwerk on my computer to produce wonderful organ sounds. Some of the samples featured elsewhere on the site include Haverhill Old Independent Church and the Paramount Theatre Organ. 

This three-keyboard stack can fully exploit some of the more interesting 3-manual Hauptwerk sample sets. Even when playing a 2-manual organ, the extra keys are not wasted, as they can be configured to operate individual stops, thereby giving a greater degree of control.  There is also a total of 24 organ pistons which can be used as combination pistons, couplers, or other organ controls. They can even carry out Hauptwerk menu commands. 

The stack plugs directly into the USB ports of a computer. It incorporates reliable, well-proven M-Audio MIDI keyboard technology, but re-packaged in a much more organist-friendly format. The stack may be used on its own, or combined with a MIDI pedalboard. Can be placed on a table, desk or other convenient surface, or built into an existing console.  The middle and top keyboards are inclined for easier playing.