Yesterday the Exchange DEV Team announced the Release of the EWS Managed API Beta. Some basic documentation is available at the the MSDN Site for the Microsoft Exchange Web Services Managed API 1.0 Beta SDK April 2009. The main object of the API is the ExchangeService. In the Getting started section You can see a basic code example for sending an email. In the future we will hopefully see some more samples and interesting results for working with the new API. David Claux has written an introductory article for the classes and methods.
TechReady 2007 Exchange Web Services Samples (Download Link)
December 2008 Exchange 2007 SP1 SDK (Download Link)
The Vista EWS Gadget (Download Link) - You can see Your inbox, calendar, tasks and read them.
The Exchange Web Services Migration Sample (Download Link)
The Exchange Developer Center (You can use this as a starting link)
The Exchange DEV Team Blog, already mentioned before (This is an alternative starting point.)
The book "Inside Microsoft Exchange 2007 Web Services" about EWS, which is currently probably the book about EWS
Glens Exchange DEV Blog - Glen has written a helper class DLL EWSUTIL, which You can reference in Powershell for various things - Glen has samples for OWA modification, contacts, finding unused mailboxes, postings to Twitter etc. - go and have a look)
MVP Henning Krauses blog (Henning maintains a project for simplyfying Exchange Push Notifications, where You get informed about what is going on in a folder and recently wrote a lot about EWS with interesting samples) .