In the last years we witnessed a proliferation of the Web APIs (Application Programming Interfaces) provided by Web companies such as eBay, Amazon, Google, Yahoo, Facebook, Twitter, etc. in order to 'open' their platforms to 3rd party developers to increase revenues. These APIs can be combined together to create new value added services called Mashups. In this paper we propose a classification of some Mashup types and then we show through examples how they can be applied to different application domains. The paper also describes a platform for Mashup creation and execution according to the Platform as a Service -PaaS Cloud Computing paradigm. Finally we describe a 'Mashup Ecosystem' enabled by such a platform and inspired by the Apple Store case study.