I'm a real fan of 37 Signal's Highrise. If you're unfamiliar, it's an online contact manager that lets you track conversations, contact information, pending communications and more.
Randy Hunt at Citizen Scholar has written a great tutorial for using Highrise with the iPhone. It's a simple process that consists of importing a vCard template into Address Book and then synchronizing Address Book to the iPhone. We love it!
Of course, you can just navigate to the Highrise homepage via Safari on the iPhone, but as Randy points out, his solution will allow you to forward emails or add to-dos to your Highrise account without launching a browser.
[Via 37signals Product Blog]