What is $\sum (x+\mathbb{Z})^{-2}$?

Another proof:

Start with the identity

$$\sin (\pi x)= \pi x \prod \left( 1-x^2/i^2 \right)$$

and apply $(d/dx)^2 \log$ to both sides.

Use residues! For an entertaining narrative with the correct answer, see here. For a derivation, see a complex analysis text.

Added

Here's a link to an outline of the residue method of solving this: Conway page 122.

As long as we're talking about the Weierstrass function, consider the parallels between the following:

1) Given a lattice $\Gamma$ in $\mathbb{R}$, the quotient $\mathbb{R}/\Gamma$ is topologically a circle. One way to describe it is to write down some functions on $\mathbb{R}$ which are invariant under $\Gamma$ and consider their image. It's natural to do this by writing down some arbitrary function on $\mathbb{R}$ and then averaging over $\Gamma$, and choosing $\frac{1}{x^2}$ gives a function which is closely related to $\sin \theta$. Now the map $\mathbb{R}/\Gamma \to (\sin' \theta, \sin \theta)$ has image the real variety $x^2 + y^2 = 1$.

2) Given a lattice $\Gamma$ in $\mathbb{C}$, the quotient $\mathbb{C}/\Gamma$ is topologically a torus. One way to describe it is to write down some functions on $\mathbb{C}$ which are invariant under $\Gamma$ and consider their image. Again it's natural to do this by writing down some arbitrary function on $\mathbb{C}$ and then averaging over $\Gamma$, and choosing $\frac{1}{z^2}$ gives a function which is closely related to the Weierstrass function $\wp(z)$. Now the map $\mathbb{C}/\Gamma \to (\wp'(z), \wp(z))$ has image the complex variety $y^2 = 4x^3 - g_2 x - g_3$. (Of course, here it is much more natural to expect that this strategy works because we know that compact Riemann surfaces are algebraic.)

Indeed, it's possible to view circles as "degenerate elliptic curves" in the following precise sense: every elliptic curve over $\mathbb{C}$ can be put in the form

$$x^2 + y^2 = 1 + a^4 x^2 y^2$$

for some $a \neq 0 \in \mathbb{C}$. The elliptic functions which parameterize these varieties are related to the Jacobi elliptic functions, and the above form is (essentially) Edwards normal form. As Edwards notes, one of the many conceptual advantages of this normal form is that as $a \to 0$, the group law degenerates to the angle addition formula for the sine and cosine! There is a paper by Franz Lemmermeyer which explores this degeneration from an arithmetic perspective.