Recent developments in the world of services on the Web show that both the number of available Web APIs as well as the applications built on top is constantly increasing. This trend is commonly attributed to the wide adoption of the REST architectural principles. Still, the development of Web APIs is rather autonomous and it is up to the providers to decide how to implement, expose and describe the Web APIs. The individual implementations are then commonly documented in textual form as part of a webpage, showing a wide variety in terms of content, structure and level of detail. As a result, client application developers are forced to manually process and interpret the documentation. Before we can achieve a higher level of automation and can make any significant improvement to current practices and technologies, we need to reach a deeper understanding of their similarities and differences. Therefore, in this paper we present a thorough analysis of the most popular Web APIs through the examination of their documentation. We provide conclusions about common description forms, output types, usage of API parameters, invocation support, level of reusability, API granularity and authentication details. The collected data builds a solid foundation for identifying deficiencies and can be used as a basis for devising common standards and guidelines for Web API development.