Stock splits typically signal a business is firing on all cylinders. This industry pioneer is no different.The S&P 500 (^GSPC 1.07%) is the most highly regarded stock market index in the U.S., comprised of the 500 largest publicly traded companies in the country. Many investors consider it to be the most reliable gauge of overall stock market performance, thanks to the breadth of its member companies.To be a member of the S&P 500, a company must meet the following criteria:Be based in the U.S.Have a market ...