user40246: I think Bitfinex did. But for example Poloniex has USDT & BTC but does not allow short selling with USDT. 

user23123: There's apparently a website that reviews various exchanges that allow trading on margin:
http://margintradingbitcoin.com/
OKCoin and Bitstamp are the two that I've heard of on there.

40246: <p>I think <code>Bitfinex</code> did. But for example <code>Poloniex</code> has <code>USDT</code> &amp; <code>BTC</code> but does not allow short selling with <code>USDT</code>. </p>
 23123: <p>There's apparently a website that reviews various exchanges that allow trading on margin:</p>

<p><a href="http://margintradingbitcoin.com/" rel="nofollow">http://margintradingbitcoin.com/</a></p>

<p>OKCoin and Bitstamp are the two that I've heard of on there.</p>
