An exchange-traded fund (“ETF”) is a marketable security that usually tracks an index or a basked of assets. Being traded as a regular stock and typically having higher liquidity and lower fees than mutual funds, it is a very popular investment option.
ETF.com has a great list of the available ETFs in the market, grouped by 1) asset class (e.g. commodities, fixed-income, etc.), 2) investment style (e.g. long-short, small cap, etc.), 3) sector (REIT, utilities, etc.), 4) region, and many other categories. It also provides a list of the ETFs that track main indices (e.g. S&P 500, Russel 3000, etc.).
This is definitely a list worth checking out: http://www.etf.com/channels