For the past month or two I've been developing a C# ActivityPub/Fediverse server called Toki, with custom ActivityPub and ActivityStreams implementations! While not having a front-end (yet), it does feature proper support for the Mastodon API set.
It's come to the point where I feel comfortable sharing it around and it's be a great pleasure for me if I could get it on the list too :)
I went with CC BY-SA 4.0 as it seemed like a good fit for documentation, but it may be a bit too copyleft as even copy-pasted snippets or screenshots can carry the license. Will need to pick an alternative.