REpresentational State Transfer (REST) has become the dominant approach to design Web APIs nowadays, resulting in thousands of public REST Web APIs offering access to a variety of data sources (e.g., open-data initiatives) or advanced functionalities (e.g., geolocation services). Unfortunately, most of these APIs do not come with any specification that developers (and machines) can rely on to automatically understand and integrate them. Instead, most of the time we have to rely on reading its ad-hoc documentation web pages, despite the existence of languages like Swagger or, more recently, OpenAPI that developers could use to formally describe their APIs. In this paper we present an example-driven discovery process that generates model-based OpenAPI specifications for REST Web APIs by using API call examples. A tool implementing our approach and a community-driven repository for the discovered APIs are also presented.