I've been a heavy user of Microsoft's network monitor. Delivered as part of Windows Server and SMS (the latter a fuller version), it's a nice tool for capturing and analysing network traffic. It's not been updated for years - allowing WireShark (Ethereal as it used to be known) to take a lead in terms of functionality.
But MS has been hard at work with Netmon 3, which is now in public beta. Network Monitor 3 is a total re-write of capture engine, parser engine, and user interface. It runs on Windows XP and Vista, Windows Server 2003 and Longhorn; and is available in 32-bit and 64-bit versions.
One of the coolest new features is display while capturing - you can see the packets as they are captured. Neat!
Thanks to Nick Portlock for pointing out this cool beta.