When Exchange-Traded Funds landed on the investment scene about a decade ago, they were alluring in that they looked to be a sexier form of mutual funds. Well, today, the ETF marketplace is exploding and this flexible investment offers tons of benefits that leave their mutual fund predecessors in the dust ... they cost less, you can short them, you can trade options on them, and you can find inverse or leveraged ones and trade them all day and night, if you want. Try doing that with your stodgy mutual funds!