Nature (the magazine, not the universe) recently unveiled their new, open, online preprint archive:
Nature Precedings
It's intended to be the natural science equivalent to the physics arXiv. But since they've been able to build it from scratch, they've included many great features. Each "article" submission receives votes, tags, and comments added by readers. This is really great to see. The arXiv seems to have built up a lot of institutional inertia, and has been dragging its feet in implementing these features, even though people have been suggesting them for years. If this new Nature site is as successful as it looks like it's going to be, the arXiv will feel a lot more pressure to implement these things. I don't imagine physicists will enjoy being out-nerded by biologists for very long.
For an example of the new site in action, the currently most popular article is this one on Open Notebook Science. Appropriately enough, it's a power point presentation discussing the advantages of doing science out in the open -- specifically, the advantages of using a wiki as a research notebook. That's no surprise to me...