Braintree Payments Opencart module is the simple and lightweight implementation of braintree.com payment service for Opencart. This extension is PCI Compliant. Currently it is the only Braintree Opencart payment gateway extension that is free and open source. It's licensed under MIT license, feel free to use it in any project or modify the source code at your will.
Browse the source code on Github: https://github.com/andyvr/braintree-payments