We live in a networked world. People are getting more and more interconnected through the new information and communication technologies, like mobile phones and the Internet. The function of cells can be understood via networks of interacting proteins. Ecosystems can be described through networks of taxonomic relationships between species. The network representation has proved to be a powerful tool to understand the structure and the dynamics of complex systems. Since the pioneering discovery of the scale-free property of the World Wide Web by Albert, Jeong and Barabasi, the study of complex...