The MidCap Spider (MDY) is a Popular ETF

The MidCap Spider (MDY) tracks the S&P MidCap 400, a group of well-established mid-size stocks that offer good growth opportunities. Like other exchange traded funds (ETFs), MDY is not managed and it has very low annual fees.

Stocks in this group tend to be a bit more volatile than larger stocks in the S&P 500 but the MDY is a good choice for the buy-and-hold investor. Combined with the SPY, which tracks the stocks in the S&P 500, you would have a diversified portfolio of large and medium size stocks.