This .NET SDK provides easy access to the Google Data APIs. It supports services like YouTube, Google Calendar, and Contacts. The Google Data APIs use the AtomPub protocol to read and write data to Google Services. Services supported include Google Calendar, Contacts, YouTube and more. This is a reference to a Google Code project.
Kasabi has just moved into public beta. The private beta is over, and Kasabi is openly ready and willing to become your data marketplace. This is similar to what the primates over at InfoChimps are doing: building a community around data sets and APIs. The Kasabi API collection feels like a first attempt at standardizing the task of designing an API for accessing a dataset.