sequences and series - Showing $sum _{k=1} 1/k^2 = pi^2/6$

I read my book of EDP, and there appears the next serie
$$\sum _{k=1} \dfrac{1}{k^2} = \dfrac{\pi^2}{6}$$
And, also, we prove that this series is equal $\frac{\pi^2}{6}$ for methods od analysis of Fourier, but...

Do you know other proof, any more simple or beautiful?