SolverLabs has just added free editions to their Angry Birds knockoff game called, Angry Pigs. They've created free versions for both Angry Pigs and Angry Pigs Seasons. I've played these free versions and the main difference is the addition of ads, which aren't really that bad or get in the way. Surprisingly, the gameplay is pretty smooth when compared to the other knockoff, Angry Farm.
Looks like OS 6 and OS 7 devices are compatible, so get updated or installed and leave us a comment on how these free versions work for you!