A lot of games have a physics engine of some sorts these days, but very few can be bothered to do something interesting with it besides make prettier explosions. The games that do make use of a physics engine for interesting gameplay tend to look like they were beat with the ugly stick. Trine manages to have fun physics-based gameplay and be one of the prettiest games to grace my system in years.
It's a 2d side scroller/beat em up/platformer where you play as a wizard, a thief, and a fighter. You bounce through spike, fire, and skeleton infested dungeons and forests in search of the generic fantasy end goal. The thief has a grappling hook and a bow, the wizard can summon boxes and planks as well as levitate objects, and the fighter hits things.
The beauty of Trine is that you can switch between characters at any point at the press of a button. Big army of skeletons? Switch to the warrior and bash them. Or switch to the wizard and summon boxes and planks over their head to crush them.