Improper integral of $\frac{x}{e^x-1}$
Make a change or variables, $e^{-x}=u$, and write the integral as
$$\int_0^\infty \frac{x e^{-x}}{1 - e^{-x}}dx$$
Now substituting gives
$$-\int_0^1 \frac{\log u}{1 - u}du $$
This is a known integral that evaluates to $\dfrac{\pi^2}{6}$.
If you want to prove it, you can use the dilogarithm. Let $1-u=x$, so that
$$-\int_0^1 \frac{\log(1 - x)}{x}dx$$
Now, since we're working on $(0,1)$ it is legitimate to use
$$\frac{{ - \log \left( {1 - x} \right)}}{x} = \sum_{n = 1}^\infty {\frac{{{x^{n - 1}}}}{n}} $$
Integrating termwise gives
$$\int_0^t \frac{-\log(1 - x)}{x} dx = \sum_{n = 1}^\infty \frac{t^n}{n^2} = \mathrm{Li}_2 (t)$$
Evaluating at $t=1$ gives
$$ -\int_0^1 \frac{\log(1 - x)}{x} dx = \sum_{n = 1}^\infty \frac{1}{n^2} = \frac{\pi^2}{6}$$
Any text book which dicusses the Riemann Zeta function will likely have this.
We have for $\Re(z) \gt 1$, that
$$ \zeta(z) \Gamma(z) = \int_{0}^{\infty} \frac{t^{z-1}}{e^t - 1} \text{d}t$$
Your integral is therefore
$$\zeta(2)\Gamma(2) = \frac{\pi^2}{6}$$
For an online discussion, see this: http://www.math.utah.edu/~milicic/zeta.pdf