Vista: Collaborate with a co-worker:

Want an easy way to share files and applications with a colleague or customer—even when you may not be part of the same network? Windows Meeting Space is a new experience in Windows Vista that enables you to start an impromptu collaboration session with other Windows Vista users. Simply open Windows Meeting Space and start a session. Windows Vista will automatically detect other Windows Vista users that are on the same sub-net infrastructure or close enough for you to create an
ad hoc (direct PC-to-PC connection) wireless connection. Once you have invited them and they have accepted,
you can share documents by simply dragging a document to the Handouts area on the bottom right which instantly replicates that file across the other meeting participants’ machines. Dragging the file to the presentation area on the left side starts application sharing, enabling the other participants to watch as you present that file. If someone has a good edit for your file, you can make that edit in real time, or pass control of the application directly to that participant for them to make that edit for you.
Source: Microsoft