According to this article, ComScore Inc. rates Wikipedia as the seventh largest website visited in the United States.  A company without an endowment and until recently, were worried they would have to sell advertising for sustainability, has just been given a donation of $3 million by the Alfred P. Sloan Foundation.  The award will come in $1 million installments for the next three years.  Wikimedia stated they would use the money for "software enhancements, including one that will make it easier for Wikipedia editors to grade article quality."