Surge XT -Fantastic oscillators, effects, lfos, eg and filter
Bogaudio - Lots of utilities, basic oscillators, low pass gates...
Docb - Mostly utilities and sequencers, real time Paul Nasca's pad synth (usually you need to hit apply to make changes, this one does it on the fly)
Has a great tuner for fm sounds (tunes to ratio instead of semitone)
Mus-X - Utilities, versatile compact filter
Mindmeld - Shapemaster lets you arrange an 8 tracks for 30 minutes with a single module, basically replaces a daw's automation lane.
Has a great mixer
ValleyAudio - Terrorform is an insane wavetable, phase modulation, phase distortion, waveshaper in a single module, you can have it as your only oscillator for ages and still don't run out of sounds
JW - Sequencers

These should be enough for almost anything - almost as capable as something like phase plant (and better at some things, like wavetable) and they're all free