do.del.icio.us
I've started a simple repository of Greasemonkey User Scripts. Mostly experiments I've been playing with in my spare time, and occasional annoyances with sites that I feel compelled to fix.
The most useful so far is do.del.icio.us - which implements a keyboard shortcut for popping the current page in Firefox directly into your del.icio.us feed. It will grab any text selected on the page and put it into the extended description field, and allows you to tag your post without needing to refresh the current page context. Just makes posting to del.icio.us that much faster and easier, though it has nothing on the coolness of their new tag suggestion functionality. In practice, I'm finding that I'm using a combination of the shortcut and the bookmarklet, so definitely see it as complimentary to the actual posting interface.
The idea came originally from pneumeric who was hassling me to make a tool for saving text selections on web pages. I also found the excellent Backpack It!, which saves selected text on a page directly to a Backpack page. Very nice; I'd like to further explore these possibilities.