Branch Cache in Windows 7 / Server 2008 R2
If I've understood it correctly this new technology in Windows 7 / Windows Server 2008 allows your computer to transparently get files from another local machine on the network rather than re-download it from a remote server.
Although it can work with a local office caching server, the really interesting bit seems to be that it supports a peer-to-peer mode, quite useful for smaller offices.