There have been a lot of exciting technologies announced this week at PDC (Atlas, LINQ, etc.) but one of the coolest ones I've seen if you're involved much with work flow applications is the Windows Workflow Foundation.  I'm happy to hear that we'll have a more unified workflow system built-into the OS (and useable on Windows XP and Windows 2003 Server):

http://msdn.microsoft.com/windowsvista/building/workflow/default.aspx?pull=/library/en-us/dnlong/html/WWFIntro.asp

If you haven't read about Atlas yet (probably one of the most blogged about ASP.NET topics over the past few days) you can read more here and even try out demos online:

http://beta.asp.net/default.aspx?tabindex=7&tabid=47