For a great example,check out what Coda is doing with Force.com and Google Docs.
Coda is a Platform as a Service partner of salesforce.com building their next generation SaaS application on the Force.com platform.
In Andy's own words:
Note that this functionality was conceived, designed and built in weeks. Let me know if you have seen this pace of innovation in the on-premise world.
Following the recent launch of the Salesforce and Google Apps offerings, we have prototyped our own CODA 2go-specific Google Apps integration! This has been built using a Google Gadget, Apex code and Visualforce and the result has got us and everyone we demonstrated it to at Dreamforce very excited about the possibilities this proof of concept opens up.
Our experience of producing finance software over decades is that accountants love manipulating transactions in spreadsheets! The success of our on-premise CODA-XL product (which extends Microsoft Excel) has been outstanding, with virtually all customer organisations adopting it enthusiastically. Now in the on-demand world we have produced an equivalent solution that uses Google Spreadsheets and Force.com.
The initial prototype can be used to perform a Cost Allocation over extracted transaction details from the CODA 2go product. The user can then apportion new values by editing the cells used by formulas in the spreadsheet; we then post back the results in the form of a journal back into CODA 2go from within the Google Spreadsheet user interface via a Visualforce-powered Google Gadget!