As it turns out, Wikipedia is not only free to use, but makes its entire database available for download, as well. I just downloaded the entire set of Wikipedia articles (not including revision history) in just over 25 minutes.
Here is a page with links to several BitTorrent trackers. Downloads seem to be possible at well over the 50Mb/sec that my cable connection can handle. Compressed, the files are still several GB, so you may not want to try this on a metered data connection.
