PocketInformant has long held the status as the most powerful PIM software on Windows Mobile on Windows Mobile. The application is loaded up with features, leaving the developer with the main task of making the complexity accessible to the casual and even power user.
PocketInformant 9 is the most recent iteration of the software, and has been released on the quiet recently. The official release is only on the 28th, but you can download the trial version from this directory here or typing this tinyurl in your mobile browser here: tinyurl.com/pipim9 .
On casual inspection the software appears much the same as version 8, but I expect on official release the changelog will be longer than my arm.