2Do is a gorgeous iPhone task management app
If you're already using iCal's built-in todo management, or Outlook's tasks on a Windows machine, and you're just looking for an iPhone app that can synchronize with what you're already using, you should check out 2Do.
2Do is a very well-designed and pretty to-do list manager, with the ability to create multiple lists, set priorities and due dates, notes, URLs, and even default actions such as Call, SMS, Email, Browse, Visit, and Google. It's a pleasure to use, with a very intuitive and pleasing interface.
2Do handles 2-way synchronizing with iCal's basic to-do functionality with the help of an additional free Sync application for your Mac, or with Outlook's tasks function with a free Windows Sync application.
I find iCal's built-in to-do management to be clunky and awkward, which isn't 2Do's fault, but does work against it when I'm evaluating what task management app to use. However, if I was on Windows using Outlook, 2Do would be my first choice as a companion task management application for the iPhone.
2Do costs $6.99US [iTunes link], and a free Lite version is available that does not support synchronization and is limited to 3 lists with 25 tasks in each list.