A "user script" or "userscript" is a program stored as plain text in a single file, usually written in JavaScript.
Similar to add-ons, extensions, and
bookmarklets, they can modify web pages and add new tools to your web browser.
How to install a user script
For many browsers, such as Firefox, a browser addon such as
Greasemonkey must be installed to use the scripts.
In the Chrome browser, scripts distributed through the Chrome Web Store can be
installed directly. Otherwise, an addon such as
Tampermokey is required.
Opera supports direct installation, but the feature must be
turned on.
To add a user script to your browser, click on its web link. Your browser will ask you to confirm that you want to install the script. Pay close attention to which websites the script will be allowed to modify.
A user script is most often stored with an extension of ".user.js". Regular JavaScript files are just ".js". For example "file.user.js" versus simply "file.js".
Saftey Information about user scripts
The most important safety tip is to pay close attention to which websites the script can access.
User scripts are used to modify and interact with webpages, and you must grant the script access to any page that it will access.
Before you complete an installation, your browser will tell you which pages the userscript can access.
You should never install scripts that ask to access your most important websites, such as your bank. Facebook and email sites are also common targets for malicious activity.
The ability of user scripts to affect your computer or browser are far more restricted than regular addons and extensions.
However, since they are more easily distributed outside of channels with centralized oversight, they should be installed with even greater caution.
Unlike a
bookmarklet, a user script runs automatically.
Unlike a standard computer program that you might install, a user script can not access files on your hard drive.
You can uninstall a script later and there will be practically no risk of hidden programs being left behind.
All of these statements are generalizations only. Like all software, you should not use userscripts from dangerous sources.
UserScripts.org
For old scripts, try the archived versions at
http://userscripts-mirror.org.
UserScripts.org appears to have died. For many years it was the number one place to find and share user scripts.
Over time users noticed that the site was slipping into a state of disrepair.
In April 2014, http://userscripts.org became seemingly permanently unavailable, however http://userscripts.org:8080 remained up for a couple of months longer.
During the interim period when the site was still available via port 8080, efforts were undertaken by various groups to create last minute archives of the site. One such effort was made public at
http://userscripts-mirror.org.
Despite the fall of such an important site, user scripts are still being created and shared. Many can be found in the Chome Web Store, on individual webpages, on GitHub and other code repositories, and on a few new sites competing to be the top replacement for the now defunct UserScripts.org.