RESTpectful comparison of Microsoft's Astoria and Google Base Data APIs
Dare has written up RESTpectful comparison of the programming models of Microsoft's Astoria and Google Base Data API for querying relational data over the web (RESTful data services).
Both these APIs also happen to support operations for changing the data - inserting, updating, and deleting - Google's details here and Astoria's in this .doc, where in addition Astoria allows associations between entities to be created and deleted). These aren't mentioned in Dare's analysis (I'm sure he knows of the ability to change the data via the APIs, not just query), however the article is worth a read nonetheless.