OSX 10.7 and later (32/64 bit), VST, VST3, AU, AAX; The plugin might work on older versions too, but it's not guaranteed. Instascale can be used on any software that supports VST/VST3 plugins with MIDI routing or AU, AAX MIDI effect type. Download free VST plugins, free synth VST, autotune VST, Drum sound VST, choir VST, Orchestra VST, and much more free VST plugins. Great place to download free VST plugins for music production, updated each week with new VST plugins check back frequently.

Effects/mixing tools and utilities/etc.
I'll start this with the recommendation that you should seek reliable sources when considering free plug ins.
It can save a lot of headaches and help find better quality plugins by sticking with reputable and known brands, and sourcing them from either the companies website or well known sources such as:
Plug in Boutique: https://www.pluginboutique.com/categori ... ?free=true
KVR http://kvraudio.com/plugins/windows/vst ... st-popular
Bedroom Producers Blog https://bedroomproducersblog.com/free-vst-plugins/
VST4Free: http://www.vst4free.com/index.php?m=VSTfx
ETC.
Beware of flaky sites and companies who often attach bloat/malware to downloads of plug ins that aren't worth even the free price tag.
Some of my personal recommendations:

Metric Scale Converter Inches

TDR Nova http://www.tokyodawn.net/tokyo-dawn-labs/
I have used this heavily since it was first introduced and am still amazed that such an amazing plug in is free!
Their other free plug ins might be of interest as well. VOS EQ, and the Kotelnikov bus compressor are both quite good too.
Voxengo SPAN http://www.voxengo.com/group/free-vst-plugin-download/

Scale Converter Calculator

Another heavily used and highly recommended tool, this is a free upgrade replacement to the frequency analyzer included in Mixcraft, with added features and utility.
Blue cat https://www.bluecataudio.com/Download/Products/
Offers some free plug in tools worth checking out. The gain suite is a useful utility.
Hofa 4U meter, fader, side pan tool is something new (to me) I am finding useful https://hofa-plugins.de/en/plugins/4u/
Melda productions free bundle contains some very useful tools that tend to be highly flexible and configurable.
https://www.meldaproduction.com/MFreeFXBundle
Also, the Tal reverb and chorus are popular and frequently recommended freebies, as is the Acon delay.
(All available via the sources at the beginning of this post.)
If guitar amp emulations are of interest, you might be interested in the free CS version of Amplitube:
http://www.ikmultimedia.com/products/amplitubecs/
It includes- a digital chromatic tuner, 9 stomps, 4 amps, 5 cabs, 3 mics, and 2 rack effects.
Some people use the stomps and effects for uses outside of Amplitube as well.