If you are referring to the desktop and not the PDA, then you can use Time and Chaos in combination with ChaosSync for PPC. You can find them here:
www.chaossoftware.com
Also, using Agenda Fusion or Pocket Informant on the PDA will still allow you to use those applications on the desktop. As far as PIM on the PDA itself, they will all use the Pocket Outlook databases, which is what the Chaos products will sync to anyway.
EDIT: Apparently the databases for WM5 don't allow 3rd party programs to access them the way WM2003 did. Chaos software does have a program that will sync Outlook to their program seamlessly, however. Sorry but that 's the closest I've seen for WM5.