I just found out about bookmarklets. As it turns out, you can mark Javascript code as a favorite, called a bookmarklet. It doesnt do malicious things because it has a small character limit. They can delete all images of a certain size, change the background color, or have a calculator.
Heres a good site for bookmarklets.
http://www.bookmarklets.com/tools/categor.html
Edited URL remove two http's