ANNANDALE, Va. (MarketWatch) -- In the midst of the market's turmoil this past week, you may have missed it but the Standard & Poor's 500 Index is marking its 50th birthday this weekend. http://www.marketwatch.com/news/sto...x?guid={36CDABB1-A3A8-435E-A494-21D9E4B0EAC9}