Our API directory now includes 46 bitcoin APIs. The newest is the Btc.to API. The most popular, in terms of page visits, is the BitStamp API. Below you'll find some more stats from the directory, including the entire list of bitcoin APIs.
In the early days of Twitter, occasionally when you tried to log in you got a “fail whale” – a picture of a whale held up by a flock of birds — as a way of telling you the system was overcapacity, and you should simply try again later. We tolerated it then because, after all, it was only social media. But there are certain types of services, for example, financial exchanges, where you never want a fail whale. So if you have thoughts of building a Bitcoin exchange, where people can buy and trade the popular virtual currency in real-time, you want an infrastructure that can handle high volume trading spikes without going down.