How can I make direct phone-to-phone VoIP calls without Internet (on LAN or Wi-Fi)? I'm looking for a way to make VoIP calls between two Android devices without having any intermediate server (SIP server) or an Internet connection. Is this possible?
While I've not tried to use my phone like this myself, I found the question interesting enough to do a little googling.  The best search term seemed to be android wifi|bluetooth walkie talkie.

The most promising free candidate I found looks to be Android Intercom - direct phone to phone communication over wifi or BT without an intermediary server or telco.  The most promising paid for app seems to be WiFi Talkie.