Some folks may not be worried about IPv4 depletion, but for those who are, there’s a neat application called ByeBye v4 for the iPhone and iPod Touch that tracks the number of IPv4 addresses remaining.
The application provides other information, too, such as the number of IPv6 addresses in use, estimated number of days before IPv4 runs out, and the percentage of TLDs using the new system.
The tool is also available as a Windows Gadget, iGoogle Gadget, and webpage widget. It has an average rating of 4/5 stars in the Apple App Store.