Ok, technically these are less bookmarks, and more bookmarklets, but I still think they are awesome. To use one of these bookmarklets:
Go to your organize bookmarks thing. (in IE or Firefox)
Add a new bookmark.
-For the "title" of bookmark, call it whatever the name of the bookmarklet is.
-In the "location" part of the bookmark, where typically you'd put the URL, simply copy and paste the huge pile of code contained in the links below.
Now when you go to that bookmark while on a page, it will execute.
On to the show!
Some of my favorites...
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Name: Kittenify
Description: Changes all images on a web page to kittens.
Code: http://pastebin.com/f61bc3fe0
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Name: Readability
Description: Makes any page readable, no matter how terrible the font and colors and pictures are, it basically turns any page into a glorified newspaper page. Very useful for myspace.
Code: http://pastebin.com/f75072e23
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Name: Javascript Shell
Description: Opens a new window in the context of the current page, and lets you execute arbitrary code on the page.
Code: http://pastebin.com/f782f9c71