You can launch iCal, Address Book or iChat from within Safari. Just type ichat:// in the address bar of Safari. addressbook:// and ical:// work, too.
[Editor's note: In testing this hint, it appears these are actually global URLs. I was able to use them in OmniWeb and Mozilla in addition to Safari. I had no luck with "iMovie://" or "iPhoto://", though. I'm not sure how exactly one would use these new URLs other than as a shortcut to get to the application when you're in the browser...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030214071907803