It could be anything from tutorials, YouTube channels, plugins/software, anything goes for this first post.
One of the most recent things I've stumbled across recently was Baphometrix's Clip-to-zero series. While I don't work on music that needs to be competitively loud, the in-depth series helped provide a new perspective to incorporate into decades-old mixing habbits.
Link to the playlist:https://www.youtube.com/watch?v=5UT42-ur080&list=PLxik-POfUXY6i_fP0f4qXNwdMxh3PXxJx&pp=iAQB (I didn't watch every episode)
I also really appreciate the work Dan Worrall is doing these days: https://www.youtube.com/c/DanWorrall
Really ? I'm no professional, but what I understand is that you don't need to buy plugins to get good sound. You just need to know how to use it properly. I saw a couple of video of people using online Reaper's free plugins.
I did use only free plugins in my last EP. Still sounds good to me.
Check Voxengo's free VST. There are great ! And SPAN is really useful. https://www.voxengo.com/group/free-vst-plugin-download/
TDR has a really nice EQ: https://www.tokyodawn.net/tdr-nova/
And Vital, one of the synth that is popular this days is free: https://vital.audio/
Oh, I agree that no one needs to pay for plugins and there's certainly a strong possibility many freeware plugins beat their paid counterparts in sound/quality.
As someone's worked professionally in music and post, workflow is important alongside sound, to me. But I still believe most people, using their ears can make equally wonderful music without dropping a dime on plugins.
That said, there are some niche plugins just aren't available in freeware. An example being Soothe2 which is my go-to transparent dynamic ducking plugin; there's no equivalent AFAIK, and the drop-dead-easy UI makes it doubly valuable when I need to unmask a sound in a mix quickly.
I used TDR Nova a little, but I didn't like the limited bands, no M/S if I recall, and the interface didn't to it for me. Sounded great though, and I can see others loving / using it (like you!).
For example, with utility EQs, I ended spending two years on F6 (after trying Nova). I liked F6 a lot more, particularly because of the M/S processing and I loved the band-solo-preview shortcut, It also cost less than Nova GE so that made the decision easy. I eventually settled on Pro-Q3, which I've now been using for years (on three personal EPs so far and almost everything else that needs EQ).
I'm very selective and only tend to buy plugins I can see myself using day in and day out or with a very specific, valuable, purpose. As an example, I bought the original Sonnox bundle for a wild price back in 2006 - but I still use those plugins to this day and they've paid for themselves ten times over on jobs over the years.
Same with my Fab Filter plugins. Q3, Saturn2 and Volcano3 get used on almost every mix.
Freeware plugins I use all the time/every mix: