The Manually Annotated Sub-Corpus (MASC) project provides data and annotations to serve as the base for a communitywide annotation effort of a subset of the American National Corpus. The MASC infrastructure enables the incorporation of contributed annotations into a single, usable format that can then be analyzed as it is or ported to any of a variety of other formats. MASC includes data from a much wider variety of genres than existing multiply-annotated corpora of English, and the project is committed to a fully open model of distribution, without restriction, for all data and annotations produced or contributed. As such, MASC is the first large-scale, open, community-based effort to create much needed language resources for NLP. This paper describes the MASC project, its corpus and annotations, and serves as a call for contributions of data and annotations from the language processing community.