At e164.org is a public ENUM service that lets people stuff their regular telephone numbers into a public name server. This enables Voice over IP calls (and more!) to be made over the Internet using a regular telephone number.
Basically, if you combine that with voxalot.com or sipbroker.com you can setup your VOIP plan to check directories first before placing a call to see if there is a cheaper/free way to make that call!