The Big Four
Each one of the big four is actually a network of independant firms who have entered into agreements with other member firms in the network to share a common name. Their agreements also set quality standards; each member is still an independantly owned and operated organization.
Here's a list of the Big Four firms
Together, the companies that make up the big four audit more than 78 percent of all U.S. public companies, according to the U.S. General Accounting Office.
Here's a listing, with links to their websites, of the top 100 accounting firms