Back in 2012/2013 there was something called "Mt Gox codes". It was a USD transfer mechanism that was supported by Gox, btc-e, early bitfinex and others. You could magically and instantly get a USD balance transferred from one exchange to the other. It was pretty crazy to see in operation as funds would instantly flow from Japan to Hong Kong to Russia/Bulgaria/wherever-btcE-was.

It was shut down by US authorities sometime in the Spring of 2013 as I recall. Tether is playing a similar inter-exchange roll today. At least now there is some kind of public ledger, but that may not be saying much.