Get lots of free samples/patchs for iOS apps or as Wav files from Electronicsounds (Donations possible).
Lots of free DX7 sysex files from Bobby Blues to use in hard and software (Dexed, Fm8…)
Native Instruments offer a ton of sounds (2000) and instruments (Synths, Smaples Instruments) plus an organizer app (Komplete Kontrol) for free. Download here.
Zombi-Fights-Shark compiled 60000 presets from Yamaha DX sysex files for Native Instruments FM8 VST synth. You can download these for free (Donation possible).
Get a couple of free (Mostly Synthwave) patches for diverse synths like Serum, U-no-LX, PG-8X and more at The Patchbay.