Firefox only: Tab Scope displays a thumbnail preview of any open tab just by mousing over it.
If this sounds familiar to the popular Tab Preview extension, that's because it is. However, Tab Scope's previews include navigation tools (back, forward, reload and stop) and support mouse-wheel scrolling. Also, you can click the magnifying-glass icon to toggle between small and large thumbnails, though you can also set custom sizes in the options menu.
This insanely useful extension has already earned a permanent home in my Firefox. Tab Scope is free, natch; it requires Firefox 1.5 or later.