I just came across this excellent introduction to and analysis of exchange traded funds from the Wharton School of Business. For the uninitiated, an ETF is similar to an index fund, but is traded more like a stock. In an index fund, Net Asset Values are calculated at the end of the day as the portfolio is values. In contrast, the values of ETFs change throughout the day. They have lower management fees than actively traded mutual funds, but buying or selling shares in an ETF involve commissions (like trading a stock).
Hat Tip to Michael Covell for the link.