What's a free or affordable collaboration solution for managing documents and files amongst multiple nontechnical users?

"Dropbox has Shared folders now between two dropbox accounts. If they wanted to control what the other could work on, they could select what goes in the shared folder. Obviously, not as good as a Sharepoint server, but it is free.

Although this wouldn't help with Quickbooks (yet), Google Docs is a great solution for document collaboration (and free). Users can see if the other is also working on the document and even send a message to the other through the Google Docs interface.

Perhaps when Google Wave comes out there will be an even more elegant solution to this problem."

Groove is also pretty decent at handling that kind of thing - comes with one of the MS Office editions - not sure which, but they may allready have it.

"I think Alfresco Cloud fits your criteria the best:

Simple: Intuitive interface to easily work, and see what is going on.
Economical: You can have 10GB without paying.
Notifications: Optionally you can receive emails about ongoing activity."

"I will recommend the solutions that I know work.

DOT NET NUKE might work for you

Visual Source Safe is one such program that will maintain version control for each user. It allows you to rollback changes and determine who edited the file and when.
Visual Source Safe

And the big grand daddy is Rational Clear Case. This is the top of the line Cadillac tool for document revision management.

Rational Clear Case Trial Download"