FYI: When SPDR S&P 500 ETF, the first exchange-traded fund, made its debut in 1993, Whitney Houston topped the charts, shoppers were snapping up Beanie Babies and Cheers was in its final season. The newfangled gizmo was an immediate success. By the end of its first year, SPDR S&P 500 (SPY), which tracks Standard & Poor’s 500-stock index, held $500 million in assets. But that was only the beginning. The number of ETFs and similar products today has swollen to more than 1,600, with combined assets of $1.8 trillion.