Firefox extension Site Launcher (download) lets you replace your bookmarks toolbar with an overlay window of links that can be pulled up at any time using a simple keyboard shortcut. You pick which sites you want on the list, and a one-letter keyboard shortcut is given to each. When pulled up, you simply hit the key for the site you want and it goes straight to it. Once you've memorized which sites are on your list, you can make use of an alternate key combination that lets you skip to each site without having to view the menu at all.
The extension comes with a handful of popular sites built in, although you can also start from scratch with your own. You also have several color and menu style options including setting the maximum number of columns you want, and how transparent the window is.
If you're a laptop or Netbook user looking to get the convenience of a bookmarks toolbar without sucking up the screen real estate, this is a great way to save some space.
Here's a quick demo video of how it works: