BBC Headliner - Makes headlines more interesting
Strip To Text - a simple script that removes all formatting from a page and reduces it to nothing but text
ComicSansify - spawn of the devil himself, Comic Sans MS - now for use on EVERY web page at the click of a button
Pig Latinise - Turns any page into a smelge of Pig-latin
Overlay an Image - Allows you to overlay a semi-transparent image onto a web page. Useful for side-by-side comparisons, or indeed, overlaying
Overlay a Ruler - Allows you to overlay a semi-transparent ruler onto a web page.
And my most complicated Bookmarklet to date is the GAME! utility bar (Details)
And finally: BBC Headline Wankery