According to everything I've read, this was a free kick and not a "free play." (And I'm not sure that a half can't end on a punt, either, unless the free kick is used.) The free kick, as above, is an unteed place-kick, drop kick, or punt, with no rush on the kicker. It is allowed after any fair catch (including a kickoff fair catch) and is mandatory after a safety.
EDIT: From the NFL digest of rules, it appears that the only way to run a play on an expired clock after a punt or kickoff is to use the free kick after a fair catch. Here's the rule: "If time expires while ball is in play and a fair catch is awarded, receiving team may choose to extend the period with one fair catch kick down. However, placekicker may not use tee."
http://www.nfl.com/rulebook/faircatch