The company which makes quadraSID also has some other interesting products, such as Slayer2, a guitar synthesiser based on physical modelling, along with amp and pedal models; it sounds almost like the real thing; not quite as authentic as sample-based systems, though a lot more flexible. The fact that it can turn randomly pressing keys into an over-the-top finger-shredding cock-rock guitar solo of the gods is impressive enough in its own right. (If I ever end up revamping a certain track from my last year's NaSoAlMo album, I may have to invest in a copy of this).
There's also this one, which seems to be optimised for goa-trance and euro-cheese, and makes making such music ridiculously easy, as well as this somewhat Casio CZ-101-ish unit.