Of Light & Shadow is one of those freeware games that could have easily been commercial offerings. It's beautiful, polished, brimming with ideas and the product of love of a sizable team.
The game, a 2.5D platformer with puzzle elements, has you playing as both Mr. Light and Dr. Shadow each with his unique abilities and weaknesses. Mr. Light is afraid of darkness and can jump around, whereas Dr. Shadow gets burned by light but can walk on walls and ceilings, making for some interesting mechanics and some quality lighting visuals.
It takes a bit to get used to the controls, but it's deliciously difficult once you finally start getting the hang of it, often requiring you to change characters mid-jump, or mid-drop.