Helm is awesome!! U-He makes a bunch of great quality instruments, another one that is really spectacular is Zebra.
It also runs on linux! Which is more or less a desert when it comes to interesting audio plugins.
JUCE is a cross-platform C++ library used by lots of synth software and VSTS plugins. While JUCE itself is open-source, there's not a lot of high-quality sample projects (that I'm aware of). Helm looks awesome, and it's exciting to see a great example using JUCE.