Notifications for Longhorn - The new method by which Windows will notify a user of information (replacing dialog boxes and the tooltip bubble that currently pops up from the taskbar:
http://msdn.microsoft.com/library/en-us/dnaero/html/wux_topic_alerts.asp

WinFS - The new file system and how it's integrated into the Shell:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnaero/html/wux_topic_storage.asp

The Longhorn Sidebar - A technical look:
http://msdn.microsoft.com/library/en-us/dnaero/html/wux_topic_sidebar.asp

Longhorn Contacts - The new way of keeping all contact information in a central location available to all applications:
http://msdn.microsoft.com/library/en-us/dnaero/html/wux_topic_contacts.asp

Typical ways of accomplishing a task in Longhorn, the easy way:
http://msdn.microsoft.com/library/en-us/dnaero/html/cb_recipe_patientdb.asp


More available at MSDN - just check the links available to the left of the MSDN website.