One of my favourite games at a bargain price for those who didn't pick it up in one of the indie bundles a few months back.
Trine is a physics-based action game where three characters allow clever solutions to challenges created by hazardous puzzles and threatening enemies. The gameplay is based on fully interactive physics - each character's different abilities and tactics can be used to invent new ways to overcome obstacles and save the kingdom!
NOT Steamworks as originally thought! My mistake, thanks who pointed that out in the comments!
Has a trade in value of £1 too which means you could pay only 49p to play the game if you wish to "trade in" your license at the end!