It's rare that an online Flash based game stands out from the crowd, but Bloxorz, a simple "fit square block into square hole" effort absolutely must be noticed. The puzzles ramp up in difficulty rather quickly, but the penalty for taking a wrong tilt isn't spirit crushing. Perfect sound, clean design, frustratingly addictive gameplay. Excellent stuff.

Bloxorz [Albino Black Sheep via Kottke]