Exchange-Traded Funds Exchange-traded funds (or ETFs) are open-ended investment companies that can be traded at any time throughout the course of the day. Typically, ETFs try to duplicate a portfolio such as SPY or the Hang Seng Index, a market sector such as energy or technology, or a commodity such as gold or petroleum; however, as the number of ETFs proliferated in 2006 from under one hundred in number to almost four hundred by the end of the year, the trend has been away from these simpler index-tracking funds to Intellidexes and other proprietary groupings of stocks.