I use Poloniex. There are some complaints lately about their service, but i'm quite happy with them so far. They charge relatively low fees (Maker 0.15%/Taker 0.25%, with less than 600BTC trade volume in 30 days, and the fees decreases proportionally if you are trading in higher volume.) Due to the surge in new active users and transactions, their website can be a bit laggy sometimes, so just be patient. yea. :D