What exactly do you mean? Trading Ethereum -> Other altcoins? Or -> USD?

If you're talking about the latter, i would just use an exchange. If you ain't, i know that shapeshift released the first cryptoexchange app, https://itunes.apple.com/us/app/shapeshift-crypto-converter/id996569075?mt=8

I have never used it, neither do i know if it works / is legit, so use it on your own risk.