I would agree with Wade here, Google Calendar. If you need phone
support (Treo, Blackberry, etc...) then you're looking at a Windows
app, but they should come with that. However, Google Calendar is just
absolutely awesome, I can't use it with my Centro myself (running Mac
OS X on my primary computer, so use iCal). However, there is NO
REASON to PAY for expensive Windows software to do what Google
Calendar does anyway.