My list of freebies would include:
Arturia's MiniFilter V, which they gave away recently. Messed with it tonight. Nice.
Arturia's MiniMoog V, a freebie from a few years ago.
Camel Audio's Camel Crusher - I've been using that one for ages.
Eventide Stereo Room, a gift from Focusrite.
Focusrite Red EQ and Comp, also a gift from Focusrite.
NI's Kontakt Player (because I'm too lazy to convert those few remaining libraries that I use to EXS-24's format).
I can't say that I use any of them regularly, but I wouldn't want to be without them.
Freebies that I used to use on the PC which aren't available on my platform:
HG fortune's synths - ProtoPlasm and STS-21 in particular. Inspiring stuff.
E-Phonic plug-ins - pretty much all of them. Tape Delay, Retro Delay, LoFi and Drumatic were among my go-to.