Iceshrimp
Iceshrimp.NET is an ActivityPub-powered decentralised social media service written in dotnet. I host an instance at dystopia.zip, running on the dev branch. Iceshrimp.NET is currently very early access, not even having its first stable release yet. Iceshrimp.NET is a rewrite of the original, less performant, Iceshrimp-JS FireFish fork (which is itself a fork of Misskey).
Iceshrimp.NET currently does not have a fully-featured frontend, but it does fully support the Mastodon API so you can connect with any Mastodon client (i.e Moshidon, Tuba, or Ice Cubes).
People I know can request a dystopia.zip account by sending me a message if I know you. The instance is (currently) not public.