In the last few years, an increasing number of massively distributed systems with millions of participants has emerged within very short time frames. Applications, such as instant messaging, file-sharing, and content distribution have attracted countless numbers of users. For example, Skype gained more than 2.5 millions of users within twelve months, and more than 50% of Internet traffic is originated by BitTorrent. These very large and still rapidly growing systems attest to a new era for the design and deployment of distributed systems [52]. In particular, they reflect what the major challenges are today for designing and implementing distributed systems: scalability, flexibility, and instant deployment.