There are a surprising number of pushable objects in Puzzle Depot, one of the most common being the humble crate.
Crates come in a variety of materials and sizes, and can be moved by either pushing or kicking them across a short distance. Crates are often used for activating pressure plates, though sometimes there are useful items inside if destroyed!
Puzzle Depot now has a dev log! Follow our progress at http://forums.tigsource.com/index.php?topic=50130.0