It's not exactly the only one, but BemaniaDX is the most popular IIDX emu at present (and the only one I've personally tried). Seems rather stable too.
Look here (might have to have an account & be logged in) to see the other programs mentioned... and if you're looking for files to use with it, you might want to start here.
I hope someone reverse-engineers the hell out of the 9th, 10th, or 11th style executables and somehow gets them running on a PC without massive turntables wired to it. I think it's entirely possible, and it would make for one hell of a IIDX sim.