A Device Panel is a set of on-screen controls (faders, buttons, knobs) that you can use to send MIDI messages of all kinds to connected external hardware. The one I designed lets you control all the knobs on my NM1. Using Device Panels you can basically turn your external synths into VSIis.
For a comprehensive list of all the major software and hardware used to create Free Range, see the above screenshot. Oh, except Blue Seal which was written in Reason and mixed in SX via Rewire, and Eye Of The Beholder which was written on a combination of SX, Logic 5.5.1 and Fruityloops, and which has all of Ott's hardware studio on it.