Helm

Helm is a free, cross-platform, polyphonic synthesizer that runs on GNU/Linux, Mac, and Windows as a standalone program and as a LV2/VST/AU/AAX plugin. It sounds absolutely enormous, has a very distinct personality, and is generally very fun to use.

The release is in beta, but given a powerful enough machine to handle the very busy UI, it is remarkably stable.

Hear a very in-depth discovery of HELM on the Open Source Musician Podcast, complete with a tour of all the preset patches.