mT gOX ALTERNATIVES

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Look here for a list of exchanges trading Bitcoin and USD: http://www.bitcoincharts.com/markets/currency/USD.html

The volume column shows you that Mt. Gox is the undisputed volume leader, with almost 10 times the size of runner-ups BTC-e and Bitstamp.
Higher volume means higher liquidity (which is good) but also allows Mt. Gox to charge higher transaction fees (which is bad).