Dulcet is a free VST Synthesizer Instrument plug-in.
Lynx is a free VST synthesizer instrument with 2 Unison oscillators with 7 voices each and 14 combined. It features a powerful wave editor that lets you draw and modify all the waveforms for both Oscillators and LFOs.
Lucid is a free VST Subtractive Synth Instrument that combines Waveshaping with FM and Morphing.
Lektro VST is a free Phase Distortion Synthesizer instrument with a lot of modulation capabilities.
Kruud is a free VST LOFI Hybrid Synthesizer. Most useful for chip sounds.
Funxion, a free Hybrid Synthesizer VST instrument plug-in for Windows with user definable functions for waveforms and distortion.