http://www.zoho.com/
Google Apps competitor.
What I really want is this functionality in a business-safe package. I am forever having to edit documents with multiple authors, and we wind up with multiple versions of the same document. But it's secret business data that I can't put on Google.
We have Sharepoint. Frankly, no one uses or understands the document editing capabilities, I think largely because the default is to download the damn file.