The Penn Treebank
The University of Pennsylvania annotated millions of words of English for part of speech and syntactic structure, and released the corpus to the community.
Why it matters
The field got a shared ruler: different methods could be compared on the same data and the same number for the first time.
The annotated material was Wall Street Journal articles, transcripts and transcribed telephone conversations. The corpus became the basis both for training and for evaluation: parser improvements started being measured as a percentage on this set. The effect ran wider than the corpus itself; it set the habit of measuring progress in language by a number on a shared set rather than by a demonstration.