GreaseKit

GreaseKit is a SIMBL plugin, that adds user scripting to Safari, Mailplane, Diet Pibb.

FreeStylerWs

Styles and skins for all popular sites in the web

Customize the way a web page displays or behaves, by using small bits of JavaScript.

Modify requests, inject JavaScript and CSS into pages.

Your Bookmarklets, On Steroids

Short-range anonymous messaging (Secret meets Firechat)

Greasemonkey compatible script manager.

Violentmonkey is a userscript manager to support running userscripts in web pages.