JUCE for Spatial Audio
Several years ago I wrote some VST plugins for Ambisonics (available here) using the Steinberg VSTSDK and it definitely wasn't particularly easy. Since then I've discovered JUCE - a framework that lets you get plugins up and running in no time. It handles all of the back-end stuff, meaning you just need to focus on the DSP and GUI.
I've already prototyped a few new plugins, just for fun, and I'm amazed at how fast it makes development. Not only does it handle the plugin "bookkeeping", it includes all sorts of modules with common tools, making coding so much easier. And on top of all that, it allows for easy cross-platform compilation! I just need to get myself a Mac to actually take advantage of that...
I wish I had used JUCE the first time around!