Duality it's very cool, and free, and never crashes (at this time on my computer). Also, the original quartz interface of MAC OSX is skinneable, but Apple not provide utilities to change the aoriginal Aqua skin.
Quartz uses PDF to present graphics on screen; it uses vector graphics fort the fonts, but all (i think) othe gui parts are rasterized graphics; it uses graphics greather than are represented on screen, in this way, the bars are always in the same size if you switches resolution.
Duality is banned by Apple, but it works very well, and their skinners make great skins.
In other cases, the major skinning works for the MAC OSX are making icons. See http://www.xicons.com
