General Market

.SPY (S&P 500) -- tracks the 500 largest publicly traded companies
.QQQQ(Nasdaq 100) -- tracks the 100 largest tech, biotech, and telecom companies
.DIA (DJIA ) -- tracks SPDR Dow Jones Industrial Average

Market Cap Indexes
. MDY (S&P Midcaps) -- tracks 400 representative madcap companies
. IWM (Russell 2000) -- tracks 2000 representative small-cap companies

Sector Indexes

.SMH – semiconductor companies
.IBB – biotech companies
.OIH – oil services companies
.HHH – Internet companies
.RTH – retail companies
.XLF – financial services companies
.XHB – home building companies