[Osiris]

This is another one of those very obvious tweaks you can do with your computer, but nobody notices them. A user on the Neowin forums took advantage of Aero Glass by placing rectangles of different colours on his wallpaper. Since his taskbar was transparent, the colours just went right through, allowing an immediate colour change for your taskbar. The final result looks something like this (click to enlarge):

Doing this is very simple. All you need is Photoshop or Paint .NET
Take your wallpaper, create a new layer, and fill the bottom portion of the wallpaper with your choice of a gradient or fill. The same could be done for the sidebar.
You probably think this is a useless trick, but it usually requires another program (that usually costs money) to make such customizations.