Here are a few I use:
Synth1, Surge, Dexed, Helm, TAL-Noizemaker, Synister, Tunefish, Odin2, OB-XD [the old Open Source one], ToneZ which is made by a member of the LMMS community, BrainStormer, Dead Duck Deducktion, Charlatan, Lynx, Exakt Lite, T-Force-Alpha Plus 2, XSRDO Analogy [an amazing modular synthesizer], Eclipsis
I recommend Synth1 if you want something very stable, you can also find tens of thousands of free presets for it if you search on the internet.
You should try the other ones too and see if they match your needs, they are all free.
Cheers!