TrueTransparency is a simple free desktop tool that brings Aero-style window border transparency to Windows XP. Or, if you already have Vista, then you can change the look of your application windows with one of five built-in skins (Aero, Aero1, Aero2, CXP and Seven for the Windows 7 look).
Need more control? The skins are based on PNG files provided with the program. Tweak these and you can make application windows look any way you like.
The latest version of TrueTransparency also brings a couple of useful new Windows 7 features to XP and Vista. There's Aero Shake, for instance - just use the mouse to shake a window from side to side, and TrueTransparency will minimise all other application windows. Shake again and it'll bring them back.
And you get support for a basic version of Aero Snap, too. Drag an application to the top of your desktop, drop it and the program will automatically maximise. Drag it back down and the window will restore to its previous size: easy.