User Script Hosting
Gist is GitHub's "paste" service, where any set of simple files can be pasted into a web form and saved.
Gist files may be named. If they are given the
.user.js suffix, then the raw link for the gist will serve perfectly as an install.
HTTPS is available and used by default.
"An open source user scripts repo built using Node.js". Open source, hosted on GitHub. (As of March 2014) HTTPS is not available.
One of the original and longest running user script hosting sites. HTTPS is available but not used/linked to by default. As of 2014, largely fallen into disrepair.
The Whole Internet
Any server across the entire web may host user scripts.
For Greasemonkey, the only requirement is that the (entire) URL ends with
.user.js and that it is not served with a
text/html content type.
Servers supporting HTTPS are best, in order to be compatible with Greasemonkey's built in update checker.