Agendus has been a longtime favorite in for the PalmOS crowd, but it looks like iambic is seeing the light. They released a version for Windows Mobile (Pocket PC edition only, though). Looks like it might be a nice competitor to the other 3rd party calendar apps out there like Agenda Fusion or Pocket Informant.
Lord knows it can't be any worse than the default calendar app in terms of usability. My thumb is calloused from all the scrolling.
Agendus for Windows Mobile optimizes to how you go about scheduling your life. From a single-screen, with a numerous View options (Day, Week, Month), you can easily schedule meetings, create (and track) tasks (AKA To Do items), write memos, create contacts, and check email