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.

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)