Greasemonkey

Greasemonkey is a browser extension that enables users to customize how web sites are displayed on their computer. Once Greasemonkey is installed, the user script enables changes in the elements of web pages. A user script can be general or specific for a particular site. For example, a user script for Google called "Butler" makes it possible for users to delete ads and add links to Google search results.

Greasemonkey was built for Firefox browser; Later, versions have been developed for other browsers, including an edition for Internet Explorer named GreasemonkIE. According to some analysts, the ability to change online content can lead to the so-called hacked web, in which case most web users are open to modification.

Install Greasemonkey and user script is relatively easy and has a security problem. An unscrupulous programmer can insert malicious programming into a user script, and the average person will not be able to detect it by looking at the code. Malicious users scripts, like  different malware, can harm a host computer or steal sensitive information among other things. The 26-year-old Seattle programmer Aaron Boodman, who created Greasemonkey, suggests that user scripts should be treated with caution in the same way as other software, and people should ensure that they come from a trusted source before install them.

In the natural world, "Grease monkey" is a slang term for a mechanic.