Given the hack on bitfinex, is there any exchanges that do not hold coins? This could prevent a large scale hack. For example, BTC/ETH exchange. After the transaction, the exchange will transfer coin out immediately.
It might only prevent one type of hack, not hacks altogether. It would still be possible to manipulate input and/or output addresses that - until discovered - could lead to significant losses for a huge number of users.
Answering your question, I think that Shapeshift.io offers instant conversion between various altcoins. However I know of no fiat exchange that is converting instantly. But withdrawing your coins instantly from the exchanges is recommended security practice since years anyway. I think the biggest losses from the bitfinex hack are most likely not associated to average buyers/sellers but to leveraged traders that kept significant funds at the exchange.
ya.ya.yo!