Web Services have experienced great interest during the last years as they were expected to play a key role as enablers of seamless application-to-application integration both within company boundaries and on a global, cross-organizational scale. As a technical foundation for the realization of Service-Oriented Architectures (SOAs), Web Services encapsulate complexity inherent to individual applications and allow for their loose coupling. However, a truly global mesh of such services has not yet become reality due to various reasons. Novel technologies and design principles are currently about to emerge which allow human users to use, customize, combine, interconnect and finally expose Web-based content or functionality as new resources which are often referred to as Mash-ups. In this article, we provide an overview of existing Mash-ups as well as tools and platforms that empower users to build them in a highly efficient and intuitive fashion. Statistical data and case studies are leveraged to examine new ways of resource provision and consumption and also the relevance of upcoming intermediaries. Finally, we investigate remaining research challenges on the path to a truly global SOA.