A New Version of Microsoft Terminal Services Client (Remote Desktop)
Version 6 of MSTSC now supports multiple monitors(!) as well as wide-screen resolutions and other features that I haven't yet tried. This is the version that comes with Vista but it's worth noting that there is a download for Windows XP SP2 and Windows Server 2003. It's also worth noting that the new features are only available if the machine you're connecting to is Vista or Longhorn Server.