Bad Piggies Free (iPad version is titled Bad Piggies HD Free) is the official lite version of Bad Piggies. Before The Road to El Porkado update, it included the first 4 levels of each story (12 for Ground Hog Day) plus 1 of their bonus levels (3 for Ground Hog Day). You could also purchase the Field of Dreams sandbox for its same price plus use Ground Hog Day’s first Sandbox (Level S-1) for free. A later update added the ability to upgrade the game to the full version for a dollar plus Toons.TV (the only time in a Rovio Free version).
When The Road to El Porkado got released, you could now watch ads to unlock the rest of the levels (which was also been applied to Angry Birds Free (first 4 only, yet their first 3 (5 for Danger Above) levels are still from the old free version) and Angry Birds Space Free). This was part of Rovio’s plan to add social features (Cake Race for Bad Piggies and the Mighty League for Angry Birds Classic) to the games plus spendable currencies (slowly added into all the AB games plus Bad Piggies since this update) to make them completely free.