RBTray - Allows almost any program to be minimized to the system tray by right clicking its minimize button.
Glass2k - Small little program that allows Windows 2000/XP users to make any window transparent.
4t Tray Minimizer - 4t Tray Minimizer Free/Pro - Minimize Outlook, Internet Explorer, Firefox, Chrome and any other applications to the system tray!
GhostWin - GhostWin - Free Download -- Transparent Windows Utility supports Windows 2000, 2003, XP, Vista and 7
Minime - Minime is a tool that allows you to minimize applications to the system tray using a simple hotkey.
Feewhee - Feel the Wheel (Feewhee) is a tool that Microsoft apparently overlooked while making Windows.