I’ve just discovered this feature in Snow Leopard and it’s changing my life. It allows me to reduce a window into the application icon (in my dock), instead of going to the right of the dock next to the trash bin. I’m really happy of that feature especially because I’m keeping the MAMP window up every time, and I didn’t like to waste space in my dock for that!
To activate this feature, go to System Preferences, then to Dock, and check the box called Minimize windows into application icon. From now on, your windows will reduce and not take more space into the dock.