Anki - I don't know how I'd prep for the MCAT without this. Open source with desktop and mobile clients, syncing via cloud. It's flashcarding but uses gradual intervals so you see things less often once you've retained them. ChatGPT - I use this as a private tutor (it's great for biomedical stuff) to check my understanding and ask it to correct me if I'm wrong.
I'm not sure who is running it (hopefully some well funded academic group), but it restores me faith in humanity a little every time I press sync and don't get CTA'd to sign up for some subscription.
Amazing tool, kudos to the devs, and if there other comes a time where payment is required, I would be happy to chip in with the above being said.