There are two separate Santa-related delights Google is offering this year. First, you can let Santa know what you want for Christmas by leaving him a message at his Google Voice number -- 855-34-SANTA (855 347-2682). According to the official Google blog, it's a free call for residents of the United States and Canada; outside those areas it will cost $.01 per minute.
If you live in the U.S., you can delight your favorite kid (or amuse your friends) by having Santa immediately send a customized voice message, complete with the person's name, state, and a variety of other customizations, which can all be combined for hilarious results (see graphic below). It's fun -- try it at Google's Send a Call From Santa site, or listen to a sample message here.
Here's the interface:
