Linguistics Professor Mark Davies has created and maintains a series of monumental corpora, including the Corpus of Contemporary American English, the Corpus of Historical American English, the TIME magazine Corpus of American English, the Corpus del Español, and the new (beta) Google Books interface. These corpora, ranging from 45 million to 425 million words (not perhaps the most widely-used corpora currently available (used by more than 80,000 people each month). They also serve as the basis for an increasing number of publications by researchers from throughout the world. The corpora have many different uses, including: finding out how native speakers actually speak and write; looking at language variation and change; finding the frequency of words, phrases, and collocates; and designing authentic language teaching materials and resources.
The corpora are usable free-of-charge at http://corpus.byu.edu