Older but still cool Reaktor synth by Brice Beasly.
Wavepeek is a free M4L synthesizer with a wave-shaping section that is meant to be easy to understand but very capable of creating interesting waveforms at the same time. It gives you full control over two sets of waveforms and morph between them using an EG, LFO, or manually. It works best when you use the shape as a filter, so make the bottom waveforms smoother and lower but you can do some other crazy stuff with it as well.
Stereo Tool is a utility plugin featuring a variety of useful processes for manipulating stereo input signals and can be used to re-balance stereo and mid-side information and place it anywhere in the stereo field. It is useful for detailed mono summing, width control, stereo balancing, and features a custom pan control specifically designed for stereo audio.
Speculum Free is a simple delay and comb filter effect plugin available in VST2, VST3 and AU formats for Windows and Mac from Deacy Bridge.