With the popularity of index mutual funds, it was just a matter of time before a spin-off investment appeared. Back in the early 1990s, the first exchange-traded fund (ETF) was created and sold. Since then, this innovative investment vehicle has grown by the billions (though they still pale in comparison to mutual funds held), more often becoming the security of choice around which to build a portfolio. Strong performance and flexibility keep these securities among investors' favorites — and more new ETFs are being introduced all the time.