Trine

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!

Source: Official webpage, "http://trine-thegame.com/site/index.php"

Trine is not completely accessible as a multiplayer co-op game. A single player can take one the roles from three different characters, but co-op play cannot be accessed through the internet nor through LAN cables, only as a local connection through the same PC or console.

"Trine has local co-op for up to 3 players. However, there is no LAN or online co-op. It will not be patched in later either - Trine is a singleplayer (+ local co-op) game and will stay that way.
Local co-op can be played in many ways. Trine supports a variety of gamepads and also has support for multiple mice and keyboards (enabled in the Launcher/Advanced).
You can find some co-op/gamepad help here: http://trine-thegame.com/help/trinegamepads.php"

Source: From the official Frozenbyte posting on Steam, "http://forums.steampowered.com/forums/showthread.php?t=1143293"

In 2014, Trine was remastered using the Trine 2 game engine and released as "Trine Enchanted Edition". This edition added online play.