The iShares Have It (State Street, Barclays, Bank of New York, Nasdaq 100 Trust, SPDR Trust)| SmartMoney.com:
"...226 ETFs currently trading in the U.S. State Street is the No. 2 provider of ETFs behind Barclays (BCS), based on total assets. In third place is Bank of New York (BK), creator of the popular Nasdaq 100 Trust (QQQQ).
ETFs are baskets of securities that track an index, much like plain-vanilla index mutual funds, yet trade like stocks throughout the day. (Mutual funds are priced just once daily.) There's plenty of good reason for investors to flock to ETFs: They're cheap, tax efficient and relatively simple to understand and invest in."