I was flipping through [Proofs Without Words](http://rads.stackoverflow.com/amzn/click/0883857006) (PWW) and saw many visual proofs for sequences and series. However, I saw none for $$\sum\_{n=1}^\infty \frac{1}{n^2} = \frac{\pi^2}{6}$$

Are there any **visual proofs** for the above series?