This is the synthesizer plug-in Surge which Claes Johanson (@kurasu) previously sold as a commercial product via the company vember audio.

As he is too busy with other projects and no longer wanted to put the effort into maintaining Surge across multiple platforms, he decided to give it a new life as an open-source project.

It was originally released in 2005, and was one of Claes’ first larger projects. It is reliable and sounds great.

surge-synthesizer.github.io