Box.net [1] filled in a big hole in its product last week when it announced an automatic desktop file syncing tool. This enables users to sync the files they choose from the desktop to the cloud automatically each time they save a marked document (and inform people they share the document with that the files have been updated). What's more, they can download files from the cloud for offline use and sync the next time they connect to the Internet. The new tool is only available for business users at this time and they will be rolling it out to existing customers over the next couple of months. For more information, see the Box.net blog post [2].
Links:
[1] http://www.box.net/
[2] http://blog.box.net/2010/06/10/introducing-box-sync-for-business/