If you use gaim (an Instant Messaging client) under XDarwin like I do, you may want the ability to click on a URL sent to you and have it open in your default web browser. To do this, you can go into gaim and in the menubar select Tools -> Preferences -> General. In the lower righthand corner, find the browser section. Change the default to "Manual (%s for URL)" and enter "open %s" in the field below. Now when you click on URL's that people have sent you they will open up in your default browser.
The 'open' program can be used to open the default application for many types of files. You may want to experiment with using this in other UNIX programs to increase the Darwin/Aqua interoperability.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021230050422489