Greasemonkey
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Greasemonkey is a software implementation of a concept called "Active Browsing". It is a free Firefox browser extension that allows altering web pages using scripts. These scripts can then be persisted, so that the alteration is permanent.
Greasemonkey can be used for adding new functionality to webpages (like embedding price comparison in Amazon webpages), fixing rendering bugs, combining data from multiple webpages, and numerous other purposes.
The Greasemonkey script repository contains a number of scripts created by people for their personal needs, and uploaded for use by others.