Clicky Gone - Clicky Gone allows you to hide your selected windows from view, this includes the taskbar button...
Glass2k - Small little program that allows Windows 2000/XP users to make any window transparent.
mapmywindows - A small program to show and hide windows in X window system.
GhostWin - GhostWin - Free Download -- Transparent Windows Utility supports Windows 2000, 2003, XP, Vista and 7
TopMenu - TopMenu is a functionality clone of ...
Feewhee - Feel the Wheel (Feewhee) is a tool that Microsoft apparently overlooked while making Windows.