HEP software today is a rich and diverse domain in itself and exists within the mushrooming world of open source software. As HEP software developers and users we can be more productive and effective if our work and our choices are informed by a good knowledge of what others in our community have created or found useful. The HEP Software and Computing Knowledge Base, hepsoftware.org, was created to facilitate this by serving as a collection point and information exchange on software projects and products, services, training, computing facilities, and relating them to the projects, experiments, organizations and science domains that offer them or use them. It was created as a contribution to the HEP Software Foundation, for which a HEP S&C knowledge base was a much requested early deliverable. This contribution will motivate and describe the system, what it offers, its content and contributions both existing and needed, and its implementation (node.js based web service and javascript client app) which has emphasized ease of use for both users and contributors.