I'm using pgf-pie. It is not a standard package. I found it on google code. As its name, it is used to create pie chart.
\begin{figure}[htp]
\centering
\begin{tikzpicture}[scale=0.9]
\pie[text=pin,style=drop shadow,rotate=240,
explode=0.2,
color={blue!70,cyan!70,red!70,orange!50}]{65/Amplificatori di potenza, 10/Elaborazione del segnale, 17.5/Sistemi di raffreddamento, 7.5/Alimentazione}
\end{tikzpicture}
\end{figure}
I would start a new line in label. In latex the command is \\ but it doesn't work. Any idea?

\\does not work in TikZ nodes by default too. You need to supply the PGF keyalign=centerto those labels somehow. – Qrrbrbirlbel Jan 04 '13 at 18:37