I was very pleased to discover that Wikipedia provides torrents of the entire English encyclopedia and that they're only about 20GB. I really thought it would amount to more than that, and I'm happy to finally have a local copy of Wikipedia. This doesn't include media like images and audio, or userpages and talk pages, but the text is by far the most critical part to have backed up.
I doubt I'd be able to host all the media at once, but this makes a totally #P2P (or #federated) encyclopedia look very possible.