Greasemonkey - Customize the way a web page displays or behaves, by using small bits of JavaScript.
Site Connector - Reuse data from one website on another.
Random User-Agent - Automatically change the user agent after specified period of time to a randomly selected one, thus...
Launchlet - Customize the web with JavaScript or CSS
Tampermonkey - Greasemonkey compatible script manager.
FreeStylerWs - Styles and skins for all popular sites in the web