Gridded

Gridded is a powerful, easy-to-use, completely free flashcard application for iOS, iPadOS and macOS. Here's how it works:
1. Select a small group of items you'd like to be quizzed on.
2. Study them.
3. Press "Play" to be quizzed on them.
4. Repeat.
Gridded features loads of ways to customize your gameplay. For example, you can choose whether to type the response or to just think the response and then grade yourself. You can choose whether diacritics, spacing, case, and punctuation are ignored. You can choose whether the items are shuffled, or whether you are quizzed in order. You can have items with multiple responses which can be accepted in any order. Or, you can have items with many responses where you are only required to type one of them.
You can choose to have Gridded speak the words on your flashcards as you play them with text-to-speech voices, and you customize how slowly or quickly these voices speak.
Gridded has a unique spaced repetition system (SRS) which uses colors to show you at a glance which items are most in need of review. When you play an item, it turns a vibrant color and then gradually gets more gray as it becomes more in need of review. Blue items turn gray quickly, within a few hours. Green items take longer to turn gray, and yellow items even longer. Red items are the ones you know best; they take years to turn gray. You can customize these colors as you like, and you can also customize the rates at which items change colors, becoming more red when you get them right and more blue when you get them wrong.
You can put images on flashcards, or mathematical symbols written in LaTeX or AsciiMath. When you import large images, they are scaled down to reasonable sizes so that you don't end up with an unnecessarily large library that takes forever to sync and share.
Syncing is easy to set up between any number of macOS, iPadOS, and iOS devices, happens automatically when you enter and exit the app, and only takes a few seconds for typical library sizes and internet speeds.
With Gridded, you have lots of flexibility to organize and categorize your flashcards. An individual item can be put in any number of decks, and multiple decks can be collected together into bundles. Every item and deck is assigned a category, and bundles can contain decks from many categories.
Categories are at the heart of one of many powerful editing features in Gridded called "Resolve Conflicts", which can be used for linking flashcards with the same prompt across multiple decks. This way, when you study such an item in one deck, your progress will be shown in the other decks as well. It only combines flashcards in the same category: if you had an item with prompt "head" in the "Spanish" category with response "la cabeza", it would not be linked to a French card with prompt "head" and response "la tête". Other tools include a built-in translator on the edit screen and the ability to split a large deck into multiple decks with more manageable sizes.
Gridded also has a "Streaks" feature which keeps track, for each category in your library, of how many days in a row, "trios" in a row, weeks in a row, months in a row, and years in a row you have studied that category. If you only have a "days in a row" streak, it can be demotivating to break your streak and be reset back to zero. If you break your "days in a row" streak in Gridded, you still have these other streaks to motivate you.
If you would like to memorize something with an order such as the digits of pi (or tau) or the lyrics to a song, you can select "New Ordered Items" and Gridded will convert these things into flashcards.
Last but not least, there is now a "Shared" tab where you can download bundles uploaded by other users and share your own bundles.