Google Docs & Spreadsheets
This plain-Jane addition to the Google empire is one of the easiest and most full-featured data and applications hosting services we evaluated. Though it doesn't yet include a presentations editor (one is on the way, we're told), it easily rates as the most trustworthy online host for documents and spreadsheets. It's free, too. All you need is a Google account.
Transferring your data into and out of Google Docs & Spreadsheets is easy. An upload system accepts files one a time and can export them in such formats as Microsoft Office, OpenOffice, HTML, and PDF. Google even provides a custom e-mail address that you can use for sending text or attached documents (no spreadsheets yet); the system then automatically converts and stores them alongside your other online documents. If nothing else, this arrangement gives you a convenient way to make instantaneous, on-the-fly backups without monkeying around in a browser. (Caveat: The service is in beta form and still has various bugs.) Gmail users can e-mail an online document as text or as an attachment without leaving the Docs & Spreadsheets system.
You can share any document or spreadsheet, and up to 50 people can work on a file at once. Though document changes aren't visible until a user refreshes the browser window, spreadsheets update in real time; this can make for some interesting collaboration scenarios as multiple users add, delete, and overwrite data simultaneously.
Google's editing tools and controls tend toward the rudimentary, and file size is severely limited--500KB for docs and 1MB for spreadsheets. A handy revisions system keeps track of old versions of your files; most competing services lack this killer feature. In addition, each user can store up to 5000 documents and 200 spreadsheets on the service.
If you need broad project management functionality, the multiple-editors-at-once feature of Google Docs & Spreadsheets probably won't work for you. There's no workflow system, no chat or discussion system, and only a basic interface that ties in well with Google's other minimalist properties.
Still, Google Docs & Spreadsheets seems remarkably stable, and its learning curve is friendly. It is arguably the most reliable and complete hosted data and applications service available.