Windows only: If you're looking to change how your Windows desktop looks and feels, CustoPack makes it easy to make either light surface paint-overs or deeper, functional shifts. It's a package-based customisation system that's easy to use, but powerful, too.
CustoPack relies on pre-packaged themes, available in the CustoPack Gallery (and, presumably, posted on forums) to fill out its selection. Pick a pack, choose the level of customisation you want to allow, then click through and let CustoPack do its work. The software, newly released by the makers of CrystalXP, is not without a few kinks here and there—I ran into an issue where CustoPack wouldn't start again after a restart with a new pack installed, but the problem fixed itself after the next restart.
CustoPack can reach very deep into your system, but the majority of design decisions are left to the pack creators. There are plenty of other tools for Windows for getting specific on what you see.
CustoPack is a free download. The app states that it works on Windows XP, Vista, and 7, but the theme packs available in the gallery, at least at the moment, seem fairly 7-centric.