Amazon does still do the price matching, and that is the only way to get your book listed as free on their site. The advantage of Smashwords is in distributing to Apple and Sony. If you go to your e-book's details page on Amazon, you will see a link that says "tell us about a lower price". You can click that link and provide them a link to the product page for your free book at one of the other sites, and eventually they will match it. Again, this seems to be more effective with Apple and Sony. The process may take a week or two, so if you can get others to click that link and share the news, it will help speed things up.
If you want to make your book free for a limited time or on a specific date, I would definitely NOT recommend this approach. There is no way to control how quickly or when Amazon will respond, so you won't be able to determine the specific date(s). Also, if you think you may want to raise the price back up after a while, I would not recommend doing this. I have one title that I published under a pseudonym that has been free for months, in spite of the fact that I have raised the price on all other sites and tried repeatedly to get Amazon to raise the price.