We've already told you that Microsoft is building the next version of Exchange to be cloud-ready and some of the cool features you can get through Outlook Web Access or Outlook Live. Exchange Web Services (EWS) is a very useful API for building custom apps against Exchange, and is even being adopted by competitors for products such as Apple's new mail client, writes Exchange team. To illustrate the openness take a look at the EWS-based Vista Gadget and the same widget for Mac OS X- a good illustration of a cross platform use of the same API. Check out video and demos below:

Additional resources:
– Exchange Server Developer Center on MSDN
– Exchange Web Services Managed API session