Userscript.Zone Search is a website that allows searching for userscripts by entering a matching URL or domain.
GreasyFork is maybe the most popular userscript hoster. It has many scripts in its inventory and is created by Jason Barnabe, the author of Stylish.
OpenUserJS started right after GreasyFork. It is created by Sizzle McTwizzle and has a lot of scripts in its inventory.
You can search Github and Gist for scripts and install them by a click at the