Ed's right, owning both hardware and software synths is a good idea. I know that Trent used an ARP 2600, for processing other sounds through, espisally on the song "Head like a Hole" from Pretty Hate Machine, and on the "The Hand that Feeds, an ARP Oddysey. Both of these synths are now virtual instruments. Arutria makes the ARP 2600V and Wayout Ware, makes the Timewarp 2600. GForce makes the Oddity which is the virtual version of the Oddysey.

On NIN's latest tour Trent has a keyboard player. Who is useing the Analogue Systems French Connection controller. Based on the Ondes Martenot, along with some small modular gear and a Korg Kaoss Pad.