Couple: an open source user-to-user chat app built on Hipmob
TL;DR: Hipmob allows you to add live support chat and real-time user-to-user chat to your Android or iOS mobile app in less than 15 minutes. Couple is a user-to-user chat application built on top of Hipmob's platform.
Hipmob provides an SDK for real-time communication that can be used to provide live support to your users (via chat) or to build user-to-user chat or group chat into your mobile or web app; it's as simple as downloading our libraries (for iOS, Android and the web), integrating them into your apps with an app key we provide and then your app has real-time communications (so you can talk to users or they can talk to each other). No server setup or code to write, just real time communication that scales with you.
Couple demonstrates Hipmob's real-time user-to-user functionality, including user identification, friend lists to control who gets to send messages to other users, launching chats between specific users and setting up push notifications for for offline messaging. You can download Couple from the Google Play store and Couple up!
We've open sourced the code used for the Android app and the Node server (hosted on Heroku) and hope it can be useful in building your own user-to-user applications.
Update: comments/discussion on Hacker News.
P.S. If you're building a mobile app and want first-class, integrated, in-app support chat or user-to-user chat you should check out Hipmob!