BlockCypher is a platform-as-a-service for crypto coins. It's exposed through simple web APIs and callbacks for developers to build Bitcoin apps. We rebuilt the Bitcoin platform from the ground-up and optimized it for high availability. Our up time is 99.99%. 

We just released our Transaction API. A transaction can be created in 3 steps:

1. With only an input address, output address, and the value transferred, we build the full unsigned transaction and the data to sign it.
2. You sign the transaction data (we do not store any private keys) and send us the signature.
3. We complete the transaction, broadcast it over the peer-to-peer network, and send it back to you.

For documentation on this and all our APIs, see http://dev.blockcypher.com/.