If you're using a keyboard without an eject key, say a non-Apple keyboard, an older Apple keyboard or (in my case) a really old Apple keyboard, you're probably missing that eject button. Sure, you can launch iTunes and select "Eject Disk" from the Controls menu, but there's a much easier way.
Navigate to the CoreServices folder, which lives in your system's Library. There, you'll find "Eject.menu" in the Menu Extras folder. Simply double-click that sucker and presto! An eject button is now in your menu bar.
To remove it, simply click it and drag it onto the desktop while holding down the Command key. You can also re-arrange menu bar items by dragging with the Command key depressed.