I have a free membership on Poker Coaching.com (Jonathan Little). It has a lot of free stuff, including quizzes, partial access to some courses, and full access to a few beginner training courses. Every few months there is full access to the entire course catalog for a few days, as an enticement to sign up for the "Premium" membership. Its possible to download quite a bit of free material during these promotional windows.
I don't play enough poker at high enough stakes that the GTO Wizard or Premium membership would be worth the cost. YMMV. I have found that the course content has shifted over the last couple of years to be incredibly GTO-centric. Where previous content was based on fundamentals (pot odds, bluff frequency, etc.), it now seems that everything is viewed through a GTO lens. That may be all well and good for higher stakes games (IMO), but it adds a lot of complexity, the need for extensive chart memorization (and there's a TON of GTO charts for every conceivable situation), and may not be the best approach for your average .25/.50 or 1/2 donk fest.