I2P and Internet anonymity
Conceptually, I2P works differently from Tor: It is not a tool to surf the Internet anonymously (although this is possible through a gateway), but a private and anonymous sub-net within the Internet. Effectively, this is an Internet in miniature with its own search-engines, email systems, blogs, file-sharing and torrent services, and similar. Unfortunately, the amount of content is still far too small for it to be a complete anonymous replacement for the Internet. Then again, the growth appears to be decent and the future may be different. (Certainly, and unsurprisingly, the file-sharing community appears to be flowering.)
Notably, the high degree of anonymity provided can be very valuable for those who live in fear of prison for criticizing their respective governments, wish to communicate anonymously within a smaller group, or similar.
As with Tor, just running a local node can be a great help to the community—and, unlike with Tor, there is no risk of landing in the eyes of the police for having relayed someone elses surfing.