Square 1 - Rubik's Cube (sorta)
I just wrote a long witty recommendation for this cube... and I accidentally hit the wrong button and it disappeared into the ether. Short version: Beautiful shapes and possibilities, maddening to solve. Geeks around the world have solved and shared their solutions for the Square 1 so you needn't leave it in a drawer like I do. I solved it once and I'm afraid to pick it back up again.