After trying many different solutions, the one that is most flexible right now has to be GetDropBox.com

It installs easily on Mac and Windows (integrating with your Mac Finder or Windows Explorer). You can then drop files onto your DropBox and they are automatically replicated on to your private web drop box.

Share the folder with someone else, and they get to see the same directory on their desktop. You can add, change and delete files, and everyone’s copy is kept in sync.

If two people edit a file at once, GetDropBox is smart enough to recognise the conflict, but leaves you to resolve it. Because you both have separate files on your desktop, you won’t be stopped from opening the file.

Up to 2Gb of drop box is free, so it is easy to test out and see how well it works for you. You can pay a small monthly fee to increase the storage.

Is there a simpler network shared drive for the cloud office? I’m always on the lookout.

Secure, fast, and suitable for lots of data.

S3 is a convenient way to backup a server. Automated offsite backups, without tapes.

S3 is also a convenient way to host large media files for websites. Not only does this cost less than expensive storage/bandwidth on our primary web servers, S3’s capacity means it can deal with high volume media serving (such as StumbleUpon or Digg events).

As noted in the Office in the Cloud diagram, you can also use S3 as a simple office file server.

So Amazon S3 is a useful tool to consider if you are looking at shifting to an Office In The Cloud.