Looking beneath the covers of the collective buzz around yesterday's launch of Google Apps for Your Domain Premium hides a new set of Google APIs that, as they describe, allow you to "integrate and extend". How? With their new Provisioning and Single Sign-On APIs.
Neither of these are 'simple' APIs in the sense of the Google Maps API, but that makes sense, these are inherently more complex operations. To begin with, it doesn't hurt to understand a bit about SAML, the Security Assertion Markup Language Authentication API used for single sign-on (SSO). Then, if you want to interactively test-out Google's SSO, you can do it with their Demo Tool.
If you look at the Google Apps API Overview, Google segments about 10 of their APIs into use by two types of users: domain administrators as users of APIs like the Provisioning API and the SSO API, as well as application developers, who'd be more likely to use APIs such as the Calendar API and the Spreadsheets API.
You can see more at the Google Apps APIs Home Page and in particular check the API overview table in the Getting Started section. In addition to these two APIs, there's also an email gateway that allows you to support archiving, dual delivery and business rules.
These APIs will start to lead beyond maps with a whole new class of mashup Integration with other big players. As in follow-on announcements like Avaya integrating VoIP with Google Apps.
In our directory there are now 20 Google APIs listed.