The World Atlas of Language Structures (WALS) is a large database of structural (phonological, grammatical, lexical) properties of languages gathered from descriptive materials (such as reference grammars) by a team of 55 authors (many of them the leading authorities on the subject).
WALS is harvested by OLAC. The first version of WALS was published as a book with CD-ROM in 2005 by Oxford University Press. The first online version was published in April 2008. Both are superseeded by the current online version, published in April 2011.
WALS Online is a joint effort of the Max Planck Institute for Evolutionary Anthropology and the Max Planck Digital Library. It is a separate publication, edited by Dryer, Matthew S. & Haspelmath, Martin (Munich: Max Planck Digital Library, 2011) ISBN: 978-3-9813099-1-1. The main programmer is Robert Forkel.