I think it is worth mentioning Microsoft Office Live, but it is a slightly different kind of online service. The storage space Microsoft will give you for free is small in comparison with ThinkFree and Google Apps, but it doesn’t use online applications, instead utilising MS Office directly from your machine. Documents can be shared and they have, for sure, 100% compatibility with MS Office.

There are possibilities to connect domains to Microsoft Office Live, in similar ways to Google Apps, for an administrator to manage user accounts for document sharing and eMail. Office Live can manage eMail for the domain with some convenient synchronisation options between webmail and web calendars and MS Outlook.

All three of these services have their different advantages and although, to me, Google’s solution stands out from the others, its lack of support for styles while working on documents raises concerns for collaborating on documents close to a final product.