i also recommend Bittrex for some decent alt coin liquidity and access to some less traded coins, like ZCL. If you want to make larger BTC purchases to get from USD to crypto, i recommend Coinbase. Once you have crypto you can trade on Poloniex, Bittrex, or similar ...but if you're looking for simple trades without an exchange i highly recommend checking out @blocktrades and their counterpart, Shapeshift.io.