INTRODUCTION: document collections (corpora) for evaluation of information retrieval (search engine) systems are pretty often behind a paywall. A notorious example is the TREC conference (http://trec.nist.gov/). Apart from money, they ask for affiliation, making the data unavailable for the hobbyist/open-source. A few older corpora exist (http://www.daviddlewis.com/resources/testcollections/reuters21578/) but these are "antique" in ways that make then less useful for the evaluation of current systems. Others like the Wikipedia dumps(1), the European Union multilingual corpora (2)(often used in automatic translation) or the RFCs (3) could be used but are not standard in the research community.
QUESTION: does any open text document corpus of a decent size and quality in English exist, that is widely used for evaluation of information retrieval systems? The widespread usage puts the result of one's own evaluation into a meaningful context.