What are /r/p2pfoundation's favorite Products & Services?

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Check out Serverless XMPP (also called "Bonjour IM" or "link-local messaging") as a starting point. It uses mDNS (a.k.a. Apple's "Bonjour") to find and automatically interact with other compatible chat clients on the same LAN.

It's supported by Adium/Pidgin, as well as iChat. Just add a new account of type "Bonjour," and pick a username you'd like to use (though, Googling it, it looks like on Windows, you may have to install Bonjour for Windows first to have Bonjour available – it should work out of the box on desktop Linux distros with Avahi and on OS X). Since usernames are self assigned, this is pseudonymous. You should see an up-to-date list of everyone else on your LAN who has Bonjour chat enabled. This is also a great way to chat if you can get on wifi but the venue's Internet connection is down.

Note though, as always, other users on your LAN who are communicating with your computer can of course see your IP and hostname, and can scan for other services running on your computer. Link-local messaging doesn't affect this negatively or positively.

I've often seen strangers on public wifi networks, especially at tech heavy places like conferences and co-working spaces. If you really want to make auto-configured local communications more feasible, I'd work on getting common chat clients to ship with link-local messaging turned on by default. That way, it would "just work" without having to bug non-technical users about enabling it.