Orbitone Three-O-Three for creamware cards is lots of fun. Not an exact emulation, mind you, but I think it's "acidity" would suffer significantly for being one.

Although most emulations allow distortion to be introduced, this also has a suboscillator slider, a filter-release slider, a filtertype switch and an overdrive button, all of which really extends the types of patches which can be made.

9 of them in unison running different patches off of the same MIDI signal makes for pretty phat, interesting acid...10 runs me out of DSPs. :)