Bit cores
One of the many alternative bitcoin implementations goes by the name of Bitcore. The project positions itself as a powerful modular node for bitcoin and blockchain-based applications. This could be quite an intriguing alternative for wallet application developers, as Bitcore implements some additional features such as additional indexes for querying address balances, unspent outputs, and the transaction history.
As one would expect, Bitcore is 100% open source and can be modified by anyone with the necessary knowledge. Some of the companies using the Bitcore implementation are Bloq, Counterparty, Streamium, Bitpay, and TREZOR. A modular service-based architecture is quite appealing after all, especially for anyone involved in enterprise bitcoin applications and services.