Lifestreams, a data storage model (use the cached download links in the top right to read the paper). This concept has two parts:
One: All your documents are organised chronologically. Incoming email will appear at the 'now' point, reminders and to-do lists exist in the future. This is the pure Lifestreams part.
Two: Folders and organisation is really done by dynamic searches, for example you could specify a substream that is 'Email from this person' and it would appear as a persistent folder with live updating. This part is used in many next-generation UI concepts.
See also: Lifestreams in Wired: "According to David Gelernter, the desktop metaphor is obsolete. He wants to move beyond space - to time."