Greenkeeper was a service designed to help developers automatically keep their npm dependencies up to date. By monitoring the changes in project dependencies, Greenkeeper would alert users when updates or breaking changes were available, allowing for smoother integration and maintenance of JavaScript projects. It aimed to improve project stability by ensuring that dependencies were current and compatible. The service was particularly useful in managing projects with numerous dependencies and frequent updates. It's important to note that as of 2019, Greenkeeper announced it would be shutting down, and users were advised to transition to other tools.