Bit Blot's Aquaria won the IGF 2007 Seumas McNally Grand Prize and clearly deserved it. It's arguably the most visually stunning 2D side-scroller you've ever seen, full of lush hand-drawn art that's like falling into a Disney cartoon, only darker and more densely textured. The game's luxurious ocean world consists of labyrinthine tunnels connecting rooms that vaguely resemble the insides of a geode, occasionally opening out into vast chambers that show off complex underwater ecosystems.
Controlling a greenish web-footed amnesiac named Naija who's searching for her family, you hold the left mouse button to swim, or move it away from Naija to make her cling to rock wall or bound off and speed away. Right-click and you can brush a ring of colored symbols with your pointer that correspond to musical notes that play harmoniously with the graceful background music. Matching specific note-colors to like-colored objects hatches collectible objects like recipes that can be employed to "cook" dishes that boost Naija's abilities like speed and defense.
Eventually you'll learn to transform into Naija's darker offensive alter ego and fire projectiles to attack enemy creatures. But while combat is both satisfying and essential, the best parts of Aquaria may just be the most downtempo ones, as you glide along secret underwater currents and explore hidden areas in search of clues to Naija's storied past.
Note: This download takes you to the developer's site