Free and probably not easy: PD; SuperCollider; Csound; ChucK

Not free: Arturia Suite; NI Reaktor, Massive, Absynth; Serum; Sylenth; Nexus; Spire

They all have a different sound. Some are modular, some are straight analogue clones, some are hybrids.

I used to design analogue synth hardware. Now I design non-analogue software. The software does a lot more, is vastly less expensive, takes up much less space, is easier to keep clean, and much easier to move. :)