The Vanish project proposes to give web users control over the lifespan of the data they post online, or to cloud computing services. Vanish encrypts your data, and all of it, even cached or archived chunks, become "permanently unreadable" at a date of your choosing, without any action on the part of the service provider or end-user.
For example, using the Firefox Vanish plugin, a user can create an email, a Google Doc document, a Facebook message, or a blog comment — specifying that the document or message should "vanish" in 8 hours. Before that 8-hour timeout expires, anyone who has access to the data can read it; however after that timer expires, nobody can read that web content — not the user, not Google, not Facebook, not a hacker who breaks into the cloud service, and not even someone who obtains a warrant for that data. That data — regardless of where stored or archived prior to the timeout — simply self-destructs and becomes permanently unreadable.