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.
Launchlet - Customize the web with JavaScript or CSS
Tampermonkey - Greasemonkey compatible script manager.
FreeStylerWs - Styles and skins for all popular sites in the web
Userscripts - An open-source userscript editor for Safari.