Continuous Integration for free

I conduct interviews and one thing a like a lot - mostly every time candidate says something I don't know. Today it was mentioned a free tools for doing Continuous Integration & Continuous Delivery:

• Bitbucket Pipelines - https://bitbucket.org/product/features/pipelines
• Heroku Continuous Delivery - https://www.heroku.com/continuous-delivery

These both have free plans and may run tests, do builds, make deployments.