Windows: Just because you're still rocking Windows XP, or you've recently picked up a netbook that zips along nicely with XP installed on it, doesn't mean you need to miss out on the polished graphics of newer Windows versions.
XP's appearance was a big step up from the boxy greys of Windows 98 and 2000, but compared to the slicker interfaces of Vista and Windows 7 it's lost a bit of lustre.
XPize is a great all in one tool for souping up the Windows XP interface. Unlike simply patching the theme.dll file—a common and old way of swapping out the appearance of Windows XP with non-Microsoft themes—XPize actually updates icons and interface components across the board. You'll no longer be stuck with some semi-modern looking icons and some dinosaur-ish Windows 98-era icon design that Microsoft never got around to polishing up for the XP release.
XPize comes pre-bundled with several alternative XP themes and allows you to not only change the main theme and enjoy newer icons, but it also allows you to do some deeper tweaking like easily swapping out the login and shut down menus. If you've been hesitant to muck around with such matters, XPize makes it nearly automatic — all you need to do is pick the look you want and reboot. XPize is freeware, Windows only.