Vanguard FTSE Emerging Markets Index Fund ETF Shares (VWO) - Stock Market Watchlist

This one is a bit risky, however, the growth potential is insane because you are being exposed to companies in emerging markets aka companies in other countries besides the US.   Having this in your portfolio with an index fund like an S&P 500 type index fund will allow you the ultimate diversification along with the huge growth potential from both indexes.