The venerable developer of Pocket Informant for the Windows Mobile platform has finally release version 1.0 of the iPhone. Pocket Informant is the quintessential Personal Information Manager (PIM) for smartphones; it allows you to manage your calendar, contacts, tasks, and notes all in one place. The major advantage of having access to all of this content in one application is that you can easily relate information between data types, like contacts to calendar items or tasks to contacts.
I have reviewed and used the Windows Mobile version for years, and was dismayed to not having access to this application on the iPhone. Unfortunately, I store all my information in Exchange and because of current technical limitations with the iPhone PIM database, WebIS has chosen to sync the calendar with Google's calendar and tasks with Toodledo. Although you can import from Outlook (my desktop client), I can't live without Over The Air (OTA) synching with Exchange. The Exchange synchronization will come, but is dependent on Apple implementing an complete API to their PIM database (maybe 3.0 has that). If you don't use Exchange I would highly recommend trying the application out.
It is a high end application at $12.99, but I believe it is well worth it (App Store link).
Disclosure: I am a Pocket Informant beta tester.