Multiparty secure computation
The basic idea is to perform a shared yet mutually private computation over the Internet. The computation could be as simple as comparing ages or as complex as an audit or an auction. In this shared computation, the inputs are mutually private, the output shared with all participants, and the privacy of the inputs does not depend on a trusted third party. For example, Alice, Bob, and Charles can each input their ages into the protocol, and the protocol will output a result indicating who is the oldest, without any of them learning the actual age of any other. They can only learn what they can deduce from their own input and the protocol's output.
Here's a good page of links to papers on the subject. There have been some very useful breakthroughs since I wrote on the subject, including orders-of-magnitude improvements in speed and security improvements in terms of how many parties must collude to fake a result or deny a party access to the output.