While others have mentioned this, I figured it was worthwhile to point out here, as there’s a lot of great tutorial information on GeoServer contained in Ian Turton’s new Open Web Mapping Course. He managed to get the content released under a creative commons license, so all are able to make use of his great work.
The other highlight is all the student projects, that are mostly built with GeoServer. They are some great examples of what’s possible with GeoServer. If anyone else has interesting sites they’ve built with GeoServer, please let us know. We’d like to start an interactive gallery soon, and these definitely make for a solid start.