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.
Hide my Windows - HMW is a free program to hide away the Windows on your desktop.
GhostWin - GhostWin - Free Download -- Transparent Windows Utility supports Windows 2000, 2003, XP, Vista and 7
RBTray - Allows almost any program to be minimized to the system tray by right clicking its minimize button.
Feewhee - Feel the Wheel (Feewhee) is a tool that Microsoft apparently overlooked while making Windows.