Quix adds tons of useful functions to your browser via a single bookmarklet
Quix is a little different than many of the marklets I'm currently using. It's a bit like the run box on your Windows computer. Click Quix on your browser's bookmarks toolbar and an entry box appears -- into which you can type dozens of commands. They'll do everything from search Google for links back to the current page, Tweet the URL, add a ruler overlay, check WHOIS and SEO information, and loads more. Have a look at the help page for a full listing.
Perhaps best of all, Quix is also extensible. Using the syntax provided, you can hack together loads of additional functionality and create custom marklets. Want to see Quix in action? Take the jump and watch the Vimeo clip!
If you're tloking for more like this, check out my previous list of 15 useful bookmarklets to power up any browser.