I've been messing around with the concept of Bookmarklets recently - small Javascript links that run from your favourites bar and do fun things on web-pages.To use, right click a link and choose "Bookmark this" or drag it to your bookmarks toolbar. When you want to use the bookmarklet, just click it like you would any other favourite.

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