GreasyFork is maybe the most popular newcomer. It has many scripts in its inventory and is created by Jason Barnabe, the author of Stylish.

• Many scripts (round about 9400 in February, 2016)
• Features like script sync from Github
• A very active open source development

OpenUserJS started right after GreasyFork. It is created by Sizzle McTwizzle and has a lot of scripts in its inventory.

• A lot of scripts (round about 3700 in February, 2016)
• Features like script sync from Github, fork scripts, ...
• A very active open source development

Userscripts.org was the source for userscripts for a long time, but the page is down now. You can find a static copy at http://userscripts-mirror.org.

• Many many many scripts (>400000)
• The original page is down and is not maintained anymore
• A lot of SPAM and SCAM scripts that for example try to hijack your Facebook account -> be careful!
• No SSL encryption