Local only posting
Local only posting is a per-post security option that lets you set whether that post can federate out to other servers or not.
Mastodon right now is designed to get your messages out to the entire fediverse. This is great, but there is a huge need for more private communities. And in a federated network I think it makes the most sense for your home server to be that community (hence "Hometown").
I've been running Friend Camp, a Mastodon instance with local only posting, for about a year. Being able to have conversations with people on your server that don't federate is a hugely liberating thing. It allows inside jokes to develop. It allows people the freedom to complain about things that they wouldn't necessarily feel comfortable leaving a trusted server (cops, employers, etc). It also lets us do things like have a server-wide movie night where we flood the local timeline with posts about the movie, and it doesn't pollute the rest of the Fediverse.
I wanted to bring this nice experience to other people, hence this very project!
For more on why local-only posting is great, check out Run Your Own Social.
There is a new icon next to the CW dropdown that looks like a chain link, either unbroken (for federated) or broken (for local-only). On each post you make, you can set whether it's local or not.
A local-only post will appear in your timeline with the broken chain icon on it:
In your Preferences menu, under "Other", there is an option that says "Allow my toots to reach other instances by default". This is checked by default, which means your posts are normally federated if you don't touch any of the per-post settings. If you uncheck this, then your posts are normally local and you must make an effort to federate your content on each post using the per-post toggle shown above.
This feature is based on the work of Renato Lond, which is itself based on a feature in the Mastodon Glitch Edition fork.