Black Pixel Land

One of my personal favorites, here look at it's features:
~ Dynamic Physics and Real-time Lighting
~ Destructible environment
~ 34 Story levels
~ Over 20 Collection levels
~ 28 different VS. levels (player vs player!)
~ Over 10 different types of Enemies (from little bitters to great priests)
~ 5 "Super Secret" Levels (mario level, pacman level...)
~ Community made levels and campaigns
~ Physics may be buggy
~ Missing internet multiplayer 

Gish can be purchased on it's homepage; Chronic Logic
You can also download demo there.
New levels and campaigns can be downloaded HERE