End-users may wish to interact with entities (e.g., groups of persons or physical objects) that need to be discovered dynamically. The entities may even reside in domains where different technologies are used. This article presents a case study on the design and implementation of a novel lookup service for the dynamic discovery of entities across technologies and domains. This novel service allows end users to dynamically discover entities that meet specific constraints (e.g., interests or abilities, availability at a specific time or location, and/or affiliation(s)). It is a powerful enabler that may be used as a pillar for a wide range of other end-user services. The proposed architecture relies on presence service and RESTful Web services. The related security aspects are studied. A proof-of-concept prototype is built and its performance is evaluated.