A somewhat-hidden feature of Address Book is its ability to add a URL to a contact. This feature used in Safari 4, which has a special Bookmarks section for Address Book entries, where it will use URLs from Address Book contact records. (For instance, if you take a look a the Address Book entries in Safari 4's Bookmarks area, you will find that the Apple Inc. card has a home page associated with it.)
It's not obvious how to accomplish this, as a Home Page field does not appear by default on a new blank card, nor when editing an old one or when adding fields using the green (+) button. The solution is in the Card » Add Field menu. From that menu, select the URL entry to add the field to any card. After entering a URL, it will then appear in Safari's Address Book bookmarks.
[robg adds: This is a somewhat obvious hint, though we've run hints on using Add Field in the past. If you'd always like the URL field to appear on new cards, open Address Book's preferences, go to the Template section, then click the Add Field drop-down menu and add the URL field to your standard template.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090611102626956