A couple for the Twitter web interface:
DeClutter Twitter: Filters out all tweets which match pre-defined keywords (iPad, 4square, etc)
Hovercard Removal: Stop hovercards from automatically appearing and blocking a whole tweet whenever you mouse over a username
Both of these are quick bookmarklets I threw together to scratch an itch, and use most days. Keyword filtering in particular is something that is supported in several twitter clients but still hasn't made it to the web interface yet, so that bookmarklet makes my timeline much more readable on Apple product launch days / during sport games I'm not interested in / etc!
Couple of simple non-twitter ones based on the url of the site you're on:
When a site you're visiting is down, check DownForEveryOneOrJustMe.com to quickly see if it's a network issue your side or not
Probably pretty niche, but do a WHOIS lookup on the current domain:
PageZipper loads multi-page articles onto one page.
I use this one, from the W3C validator, very handy for web developers:
Visual Event to show all attached event handlers
While Opera has an option to automatically reload a web page at pre-defined intervals, Internet Explorer, Firefox & Safari do not have any such feature by default.
Here's my version of a bookmarklet to automatically refresh any web-page after a specified interval. This is useful for tracking web-pages that serve info dynamically but may not have implemented AJAX to auto-refresh.
Universal Wishlist - Whimventory