Well, all sets specify the nominal impedance if you dig deep enough into the manual/specs.

Image Dynamics Chameleons are available in both 2 and 4 ohm models.

I believe the nominal rating on the BA pros is 3 ohms if I remember correctly.

The older Orion HCCA comps were also a funny load, either 2 or 3 ohms. These were the red basket sets that look exactly like an a/d/s/ 3 series set.

Soundstream used to make thier Exact series of components in 2 ohm as well.

Those are all of the ones I can think of right now....hope that helps.