The BlockTrail Wallet API is a 2of3 Multi-Signature HD Wallet API that provides easy to use wallet functionality with the advantages of having a HD wallet with a secondary backup key and a blocktrail co-singing key.
While (with the SDKs) still remaining easy enough to use for any developer to integrate bitcoin functionality into their applications without any prior (technical) bitcoin knowledge.
Following is a draft of the 'spec' of how the various aspects of the wallet function under the hood.
PHP Implementation of the Client SDK: https://github.com/blocktrail/blocktrail-sdk-php
NodeJS Implementation of the Client SDK: https://github.com/blocktrail/blocktrail-sdk-nodejs/tree/wallet-api