Placetrack + Google Latitud (Not really free)
I don't know of any "free" option ($1.99), but for less than the price of 1 beer you can get PlaceTrack and it will run in the background and you can track your phone using Google latitud, I got it, and it works pretty well, however consider that it depends on Cell towers and Google maps, so there could be some places there the accuracy could go from a few feet to a couple of miles.
Also note that it will run in the background, and even when it only activate when you move, if you really wanted to replace "find My phone" it will affect your battery life considerable.