Freenet is a fully distributed, peer to peer anonymous publishing network, offering secure ways to store data, as well as some approaches attempting to address the loads of a flash flood. While Freenet is designed as a distributed data store, people have built applications on top of it to do more generic anonymous communication, such as static websites and message boards.
Compared to I2P, Freenet offers some substantial benefits - it is a distributed data store, while I2P is not, allowing people to retrieve the content published by others even when the publisher is no longer online. In addition, it should be able to distribute popular data fairly efficiently. I2P itself does not and will not provide this functionality. On the other hand, there is overlap for users who simply want to communicate with each other anonymously through websites, message boards, file sharing programs, etc. There have also been some attempts to develop a distributed data store to run on top of I2P, (most recently a port of Tahoe-LAFS) but nothing is yet ready for general use.
However, even ignoring any implementations issues, there are some concerns about Freenet's algorithms from both a scalability and anonymity perspective, owing largely to Freenet's heuristic driven routing. The interactions of various techniques certainly may successfully deter various attacks, and perhaps some aspects of the routing algorithms will provide the hoped for scalability. Unfortunately, not much analysis of the algorithms involved has resulted in positive results, but there is still hope. At the very least, Freenet does provide substantial anonymity against an attacker who does not have the resources necessary to analyze it further.