I have to slip in a small section detailing one of the coolest pieces of open source software I've seen this year. The Java Apache Mail Enterprise Server (James) is a truly clever bit of development. Just as servlets sit behind a web server and get to react to requests from a browser, James has "mailets" that sit behind its mail server and react to specific messages.
This is a brilliant technology. You can write a simple class or two to intercept email leaving your organization, and do things with the emaillog it, search it for industrial secrets, translate it into Hungarian, encrypt it, whateverand then allow the email to go on its way afterwards, replace it with a new version, send it somewhere else, copy it to other people, or just kill itthe options are endless.