Used to use Outlook as it merged well with my email, contacts etc.
But changed laptop and no longer have full office or Outlook - and not going to pay the silly money Bill Gates wants for them either.
But as I use Firefox as a browser I downloaded the Thunderbird email program by the same people (Mozilla) - there is a very handy add-on for T/bird which basically gives it a calendar, tasks/to-do lists etc... Very similar to Outlook.
Now I just use that.
However, if you want it to synch to a mobile, or PDA then you MIGHT be shit out of luck as they tend to be set up to synch with Windows mainstream utilities. The Thunderbird one does - I believe - synch with Nokia mobiles (or can be set-up to do so), dont know about the others.