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The Web of data promotes the idea that more and more data are interconnected. A step towards this goal is to bring more structured annotations to existing documents using common vocabularies or ontologies. Semi-structured texts such as scientific, medical or news articles as well as forum and archived mailing list threads or (micro-) blog posts can hence be semantically annotated. Named Entity (NE) extractors play a key role for extracting structured information by identifying features, also called entities, and by linking them to other web resources by means of typed inferences. In this article, we propose a thorough evaluation of five popular Linked Data entity extractors which expose APIs: AlchemyAPI, DBPedia Spotlight, Extractiv, OpenCalais and Zemanta. We present NERD, an evaluation framework we have developed and the results of a controlled evaluation performed by human beings that consists in assigning a Boolean value to three criteria: entity detection, entity type and entity disambiguation.