There has been a guide to Tor hidden services for a long while now. It was about time for an I2P guide. For reference, here's the latest Tor Guide for Hidden Service (V. 4.0) http://pastebin.com/zRLGDRCM

The Invisible Internet Project (I2P) is an overlay network and darknet that allows applications to send messages to each other pseudonymously and securely. Uses include anonymous Web surfing, chatting, blogging and file transfers. The software that implements this layer is called an I2P router and a computer running I2P is called an I2P node.

The software is free and open source and is published under multiple licenses. The name I2P is derived from Invisible Internet Project, which, in pseudo-mathematical notation, is represented as I²P.