In fact, this shouldn't be too difficult to build. I don't know much about Freenet, but I
do know that if we can use bittorrent to share files using P2P, the same can presumably be done with websites. The most immediate problem that comes to mind is that, while the files being shared over P2P stay fairly constant, a website is more dynamic and gets updated a lot. I presume this could be solved by a PGP-like web of trust.
I'm sure there are plenty of other difficulties to deal with I and other can think of if I spend some more time thinking about it, but I think this would actually be a pretty neat thing to build. The great thing about this would be that the more likely something is to be censored right now, the less likely it is to be censored on something like this - more people would visit/download such things, thus creating extra nodes for other people to visit/download from.
EDIT: I just thought there would be an issue of privacy here that needs to be solved first. For example, if you can track the IP addresses of the people you're downloading your 'website updates' from, and you're visiting explicit sites, it becomes easy to tell whom of your friends are viewing those. ;) The easiest solution is to let people decide for themselves which sites to share, of course, but I wonder if this would mean many sites would be excluded, or whether the effects of this would be minimal.