The so-called API economy is pushing more and more companies to provide open Web APIs to access their data, typically using the JavaScript Object Notation (JSON) as interchange data format. While JSON has been designed to be easy to read and parse, their structure is implicit. This poses a serious problem when consuming and integrating Web APIs from different sources since it forces us to manually analyze each individual API in detail. This paper presents JSONDiscoverer, a tool that alleviates this problem by discovering (and visualizing) the implicit schema of JSON documents as well as possible composition links among JSON-based Web APIs.