The mda-vst collection of effects has been a long-time favorite for me. It’s a set of no-nonsense, unique, simple effects, just useful stuff that doesn’t have any unnecessary bells and whistles. Oh, yeah – and it’s free, making an easy way to fill out your effect arsenal. But until recently, the collection was proprietary freeware. Now, it’s GPL-licensed open source for Mac and Windows.
Included: multi-band distortion, drum replacement, amp and speaker simulators, de-essing, degrading, delay, detune, dither, dub delay, compressor/limiter/gate, envelope following stereo imaging and simulation, a Leslie simulator, multi-band compression, an overdrive, a really insane pitch changer, a 3D panner, a sub-bass synth, a couple of vocoders with different numbers of bands, test tone creator, flanger, pitch tracker, and more.
I imagine the access to code for these things could help people launch their own effects projects. And as Windows VSTs, it can run easily in Linux hosts that support that format, too.
Thanks to Marc Resibois for the tip. And you budding C coders out there, if you dig into the code, let us know.