Helm

Helm is a free software synthesizer. It is used to create electronic music on the computer.

Helm is free. This means that you control this software, it does not control you.

Any sound that comes out of Helm belongs to the person who played it.
You are the copyright holder of any sound you create with Helm.

Helm is available as VST, AU, LV2 and standalone.
Operating system requirements:
Windows 7 or later
MacOS 10.7 or later
Ubuntu Trusty 14.04 or later
Debian Jessie or later