These corpora were created by Mark Davies, Professor of Linguistics at Brigham Young University. They 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 used by more than 100,000 people each month (more than 200,000 visits), which makes them perhaps the most widely-used corpora currently available. They also serve as the basis for an increasing number of publications by researchers from throughout the world.
In addition to the regular corpus interfaces listed below, there are also many new COCA-based resources, such as www.WordAndPhrase.info, www.wordfrequency.info, www.ngrams.info, and www.academicwords.info, all of which allow you to download large amounts of corpus data for offline use. Note especially the new 100,000 integrated word list from COCA, COHA, BNC, and SOAP — the largest, corrected frequency list of English.