Those interested in immutable data-structures might like to look into Irmin [1], git-like distributed storage (due to be released soon). It's also part of a larger stack [2].

[1] http://openmirage.org/blog/introducing-irmin -- prev discussion: https://news.ycombinator.com/item?id=8053687

[2] http://amirchaudhry.com/brewing-miso-to-serve-nymote/