Microsoft has a new set of APIs that expose services across the whole of Office 365 in a RESTful way, allowing developers to consume Office data and services from any application. 

Announcing the new opportunities for developers around Office 365. Arpan Shah, posted:

Productivity apps should be available everywhere and anywhere users want them. The average information worker spends hours each day in the Office user experience across Office apps, web browsers, and mobile devices. We’re committed to enabling developers to easily and securely build solutions that integrate across the entire Office 365 user experience, including core productivity,mail, social collaboration, and communication powered by Office, Exchange, SharePoint, Yammer, and Lync.

The Office 365 extensions use open standards: HTML, CSS3, and Javascript for user interfaces; and REST, OData, and OAuth for secure access to data and services. This should permit easy refactoring of existing solutions as well as the straightforward creation of new ones.

Shah explains:

We are moving in a direction where you won’t have to develop against SharePoint and Exchange-specific APIs; you will be able to develop against a consolidated Office 365 API set that spans SharePoint, Exchange, Lync, and Azure AD. We’re exposing Calendar, People, Mail and File Office 365 APIs that make interaction with the platform intuitive and easy. One of these platform APIs is the Discovery Service APIs, which allow you to determine the location of related Office 365 services for particular users, including Files, Calendar, and Mail APIs.

The announcement also points out that the use of open standards such as HTML5, REST web services, and OAuth means that developers can choose which development tools, platform, and languages to use - including Ruby, C, .NET, Java, Python, PHP, and others.

For those of you prefer Visual Studio and Azure as their primary development environment and hosting platform, a Cloud Business Apps project template for rapid development of business apps for SharePoint that extend Office 365 has been provided.

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This release seems to be just the start of working with Microsoft's cloud productivity ecosystem. According to  the Overview of Office 365 APIs Preview: 

The preview release of Office 365 APIs supports a limited number of SharePoint, Exchange, and Windows Azure Active Directory (AD) data sources. But this is only the start of a much larger innovation. Moving forward, we’ll also support additional data sources in SharePoint and Exchange as well as supporting additional services like Yammer.