TransWin helps set transparency to windows. Unlike Windows Vista/7 Aero, it can set the transparency to the whole client area of any windows. It can also set transparency to task bar, start menu, pop up menu etc. It can be used to set transparency under Windows 2000/XP too. The transparency ratio is controllable, and it supports filters to let you control the transparency for special programs and windows.
||Free to try
Windows Server 2008