I just found something new for a pi (or similar hardware). https://piratebox.cc/faq
It creates an offline, anonymous file sharing, chat room, forum, and optionally an irc server. What it does is set up a wifi access point that anyone can join, and it redirects web requests to the local page where everything is located. As it's not connected to the wider internet, it is naturally secure, private, and local only. They are also working on mesh network support as well.