Just re-discovered Google Play after they opened their new Music All Access offer in EU, and saw this on their front page: "Shop for millions of songs from your favorite artists, including hundreds of free tracks from the biggest names in Rock, Pop, Hip-Hop, and more."
So I wonder how to find the free tracks available?
There is the "Deal of the day" (every day a new track for free): http://play.google.com/store/music/collection/deal_of_the_day
Also found another URL in several blogs/posts, entitled "GET THE HOTTEST SONGS, FREE ON GOOGLE PLAY!" http://play.google.com/store/music/collection/series_overview_theme_2011_10_29 But it's empty now.
PS: there is (was) a blog called "Magnifier" on Blogspot that somehow promoted such tracks, but it's not been updated since late December 2012, as they said they have been absorbed by Google Play.