Live Documents is a free service currently in Beta that allows Windows users to synchronize their Office documents like Word and Excel over the web with other users via a toolbar.

The creator of a document sets permissions of other users (read only, read-write, print permission, etc.) and emails out the file to them as they normally would. When the other users open the document all instances of the document across all users are syncronized regularly when users are online.

Via Techcrunch