In addition to MS Sharepoint Services, another option could be something like Intranets.com. For example, see:

http://www.intranets.com/ProductInfo/Intranet/Application/Document_Management.asp

Again, this provides an environment for checking documents in and out.

If you want a true, online spreadsheet, you may want to look at something like "Roberts Online Spreadsheet" at http://www.intrepid.com/~robertl/spreadsheet1.html

I expect online applications (including spreadsheets, word processing, and much more) to become much more pervasive. Online applications are still in their early stages. Salesforce.com, online ERP, many blogs, content management systems, etc. provide early examples. I'm sure that more is sure to come.

For some projects, I use a free email account from Google Gmail and use the web-based interface to write notes to myself. I can make notes for myself in this way from any web connection. When I'm ready, I can then copy and paste the content from Gmail to my word processor.