Here is an amusing extension for Firefox. If you are in the habit of opening dozens of tabs in Firefox and then forget about some of them, then Aging Tabs will help you identify which tabs your aren't paying attention to. The tabs that you haven't clicked on for some time will gradually fade to the color of your choice, becoming darker and darker with passing time. After a few moments, your tab bar will be filled with tabs of various shades of color and you can tell at a glance which tabs have been open and untouched for a long time by it's color. You can then either choose to return to the tab that is being neglected or simply close it to free up space.
Give it a try. It's sort of fun! (Note: Aging Tabs currently does not support Firefox 3)