I'm not one to stat the obvious but... evox
and skinning
I vagually remember something called Evolution-X
(actually evox is the bios counterpart) but that didn't include much skinning... it was more a matter of choosing different background pictures.
MXM was the first to have support for what you can truly call skinning (first dash that is, xbmp also had skinning, amongst others).
I feel currently UnleashX and MXM are the only dashes that you can seriously call skinnable dashes (other dashes like avalaunch, evolution-x and thc offer options for different pictures to be used but all your controls are static)
Oh, and Hectobleezy:
I completely left out your skinner here, sorry bout that. But I want to be sincere and I can't judge your program because it's not public yet and therefor, unfortunately, I cannot test it and judge it