Came here to say this. Anki is one of my most-used apps, been using it for years and it's rock-solid and amazing. It's open source and completely free, except

Came here to say this. Anki is one of my most-used apps, been using it for years and it's rock-solid and amazing. It's open source and completely free, except for the iphone version, which is their only source of revenue.