Gradle is an open-source JVM build automation tool created by Hans Dockter in 2007. As the default build system for Android, it powers 20M+ app builds worldwide with Groovy/Kotlin DSL and incremental builds.
Jenkins originated from the Hudson project in 2005 and became independent in 2011. It is the world's most popular open-source CI/CD server with a 1000+ plugin ecosystem and Pipeline as Code.
Maven is an open-source Java build and dependency management tool released by Apache in 2002, known for POM and convention over configuration, with Maven Central hosting millions of artifacts and 1B+ monthly downloads.
Buildkite is a hybrid CI/CD platform founded in Australia in 2013, letting 3000+ teams run builds on their own infrastructure with self-hosted agents plus cloud orchestration, balancing flexibility, security and low ops.
Travis CI is a hosted CI service founded in Berlin in 2011 and one of the first to deeply integrate with GitHub, automating build and test via a single .travis.yml file, free for open source.