Looking for something like Surge XT? Here are 6 synthesisers alternatives, free and paid, ranked by score.
Free FM synth and Yamaha DX7 editor/emulation.
Free analogue-modelled synth inspired by the Oberheim OB-X.
Free, CPU-light subtractive synth from u-he.
Free two-oscillator analogue-style synth from u-he.
Free spectral wavetable synth with a genuinely complete free tier.
Free single-oscillator taster of u-he's flagship Zebra2.