An in-house build server with over-the-air distribution for iOS and Android that combines the Jenkins CI server with custom build scripting and a web frontend for over-the-air distribution of iOS and Android binaries.
I developed this as a way to provide easy access to up-to-date internal builds of all of our applications without developer intervention. This massively improved developer productivity and alleviated stress before performing releases as the build system could guarantee proper builds with correct settings.
Written primarily in Go and JavaScript.