http://freenet.sourceforge.net/What is Freenet?
The Freenet project aims to create an information publication system similar to the World Wide Web (but with several major advantages over it--see next section). Information can be inserted into the system and associated with a "key" (the key is normally some form of description of the data such as "freenet source code V1.0"). Later anyone else can retrieve the data using the appropriate key. In this respect it is a little like the World Wide Web which requires a "URL" to retrieve a particular document.
To participate in this system users will simply need to run a piece of server software on their computer, and optionally use a client program to insert and retrieve information from the system. Anyone can write a client (or indeed a server), however the reference implementations will be written in Java.
Why is Freenet interesting?
Freenet does not have any form of centralised control or administration.
It will be virtually impossible to forcibly remove a piece of information from Freenet.
Both authors and readers of information stored on this system may remain anonymous if they wish.
Information will be distributed throughout the Freenet network in such a way that it is difficult to determine where information is being stored.
Anyone can publish information: they don't need to buy a domain name, or even a permanent Internet connection.
Availability of information will increase in proportion to the demand for that information.
Information will move from parts of the Internet where it is in low demand to areas where demand is greater.