iCal, Mail, and Address Book have become much more integrated in Tiger than they were in Panther. It has even become possible to make a very simple Customer or Contact Relationship Management system using just these three apps.
On this Mac OS X Tutorial page, I've explained how this works. It's not meant as a replacement for Daylite or another heavy-use CRM program, but for an individual user, I think this goes a long way...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050601111355907