While most of my colleagues use Outlook or Entourage, I really prefer Apple's solution: iCal, Address Book, and Mail. This really does everything I want/need. These apps work together and throughout a number of Apple applications.
iCal
One of the things I love about iCal is the fact that I can maintain multiple calendars in one place. I have at least 10 different calendars. For example, I have a calendar for my classes and my office hours, trips, special days (birthdays/anniversaries), etc. To further make me even happier the calendars can be color-coded. You can also add multiple alarms/alerts to an event.
Address Book
Address Book is a nice simple contacts management app. It does a great job with contacts and multiple addresses/email addresses/phone numbers, etc. It is a little "clunky" when dealing with family addresses (i.e., Mom & Dad). Tehre are creative ways to make it work, but it's just a little bit of a pain.
Mail.app
Managing all the email we get can be a big problem. Apple's Mail.app takes a lot of the hassle out of managing multiple email accounts. I use it with POP3 and IMAP accounts. All my messages end up in one place.
Syncing
I sync everything between my office iMac and my laptop, my iPhone and my iPad using MobileMe. While Microsoft Outlook and Entourage are pretty well known and used in the enterprise setting. The latest versions of iCal , Address Book, and Mail.app work with Exchange Server out of the box.
