For whatever reason, the "Safely Remove Hardware" dialog always seems a bit less than convenient to get to for me—mine is usually hidden behind that expanding arrow, and clicking through all the right menus when I just want to yank the cord is a bit tiring. The How-To Geek has a salve for impatient folks like myself: A hot-key-assignable shortcut to the full menu, with a "Stop" button close at hand. For those handy with shortcuts, here's where you should point one:
RunDll32.exe shell32.dll,Control_RunDLL hotplug.dll
Otherwise, hit the link for more detailed instructions. For those who do a lot of USB-swapping, having an eject dialog at, say, Windows+E might come in really handy.