NomaDesk looks like a pretty comprehensive stab at the collaborative/document sharing process for small businesses. It bumps up against other web-based services such as Basecamp (which we use pervasively at MyST) and Central Desktop (which I know very little about).
One business requirement that NomaDesk meets head-on is security - their industrial-strength 128-bit encryption and Delta-sync™ technology is pretty cool (and highly necessary). Only the changes and updates for your documents are distributed across all NomaDesk workspaces so your team-documents are 100% safe and always available, all of the time and from everywhere.
Take it for a test drive while it's free and give me a shout if you have any comments.