Good, new technology: OpenCola Folders. Two parts, the technology and the UI. Technology: Documents can be like other documents. You see a document, you can see documents like it. This is done with distributed databases, spiders to find similar documents (documents can appear anywhere on the net) [technical overview]. UI: "Create a folder on your desktop. Drag a file into that folder. The folder automatically fills up with other files that are similar. If you take something out of the folder, it doesn't find stuff like that anymore. You can create as many folders as you want by setting them up yourself or by adopting them from other users." [source]
That's the best interface for live searches I've heard of so far. How about this in the OS? And, in fact, why can't this be done right now, in email clients? There are enough links between individual messages and people, including (but not limited too) people on the same cc: list (strong link), people you were talking to around the same time (weak link), messages which have "similar" subjects/bodies (weak link), between people you forward messages between (strong). When the volume of email in the archive becomes extremely large, a fuzzy live "emails like this" folder would be very powerful.
And before anybody else says it: Intertwingle (JWZ's hypothesised email application that works along these lines).