I read about W3C’s project Memento a while ago but its become a reality recently.
The Memento protocol is a straightforward extension of HTTP that adds a time dimension to the Web. It supports integrating live web resources, resources in versioning systems, and archived resources in web archives into an interoperable, distributed, machine-accessible versioning system for the entire web. W3C finds Memento work with online reversion history extremely useful for the Web in general and practical application on its own standards to be able to illustrate how they evolve over time
Its smart, simple and great because it works on top of http, instead of creating a whole different way of doing the same thing.
I can already imagine memento powered twitter service or memento powered BBC redux service.