What exchanges offer trail orders?
This answer needs an update. As of 2017, the following exchanges support trailing stop orders:


Kraken - fees up to 0.16% - 0.26%
Bitfinex - fess up to 0.10% - 0.20%
Bitstamp - fees up to 0.25%


However, there's an important warning to be heeded: by placing trailing stop orders with any exchange, you expose yourself to very short-lived price spikes that are not reflected anywhere else. Here is a screenshot from Kraken where the three red dots represent trades triggered by the price falling by 25% for a minute, only to shoot back up immediately afterwards. The trailing stop didn't offer any protection here; on the contrary, it caused a loss.