I’m proposing a law which states that if your application supports any concept of tabs, that users must be allowed to select tabs using some accelerator key and a number. On the Mac, it’s usually the Command key and a number, and on the PC, it’s ordinarily the Control key and a number. For example, if you’re using a Mac and you have a Firefox window open with five tabs, you can go to the fourth tab from the left, you can just hit Command-4. (On Windows, Control-4.)
There are a number of applications that already support this convention, like Firefox, TextMate, and Adium. Eclipse and Trillian are the applications that have annoyed me most recently by not supporting it. The lack of support for this convention is what kept me from switching from Firefox to Camino as my everyday browser. (Safari uses the convention to select bookmarks in the Bookmarks Bar. That strikes me as particularly broken.)
Once you’ve gotten used to dealing with tabs in this fashion, you’ll never want to navigate them any other way. I find that even if I am too lazy to count the tabs, just guessing until I hit the right one is still faster than reaching for the mouse or figuring out which keys the application I’m in right then uses for switching to the next tab.