Coinbase Multisig

Coinbase Multisig API allows developers to create multi signature accounts using the Coinbase platform. Each multi signature account internally keeps track of multiple accounts, with the multisig API, developers will be able to list, and manage their accounts under one umbrella in a hierarchical manner.

Toshi

Written in Ruby and backed by PostgreSQL, Toshi is a free API for Bitcoin app developers. The Bitcoin node, which is fully compatible with Bitcoin Core, provides a JSON API to query raw blockchain data. This enables developers to read information about blocks or transactions, broadcast new transactions, or get blockchain stats like height and confirmation times.