Download free presets for your favourite synths at Presetshare.
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.