CentralDesktop

We're users of Centraldesktop - in our opinion it's one of the richest collaboration environments around. They've just announced an increase in storage capacity for the paid-for plans:

We use Centraldesktop to collaborate both within the business and with clients - it's worth using purely for the ability to share and comment on documents from draft stage to final approvals. Once you've tried the service you'll really wonder why you persisted with emailing copies to each other for so long.

Have a look here at their overview of the features that are included - with price plans starting at zero you can't go too far wrong signing up and trialling the service in your business.