I hate having to take my hands off the keyboard, like when I reach for the mouse to switch between tabs in my browser. It's particularly annoying when I have a bunch of tabs open and the two I'm hopping between are separated by a bunch of other tabs.
Enter FLST, or Focus Last Selected Tab, a Firefox extension that lets you hop between any two tabs using a simple keyboard shortcut (Ctrl-period). Alas, you can't customize the shortcut, but I find it extremely easy to hit while typing along.
For months I've been dismayed that FLST wasn't updated to support Firefox 3, at least according to Mozilla's add-ons site, but the version available directly from the developer's site (linked above) works just fine.
Now, savvy Firefox users will note that the browser has a built-in solution that's nearly as good: Press Ctrl-Tab to switch to the next tab, Ctrl-Shift-Tab for the previous tab. But that assumes the two tabs you're using are side by side. Sure, you can drag them next to each other, but that's an extra step. And you have to remember whether you need to go forward or back. With FLST, the same key toggles you between your two most recent tabs. For bloggers like me, that's crazy-handy.