SuperMemo

SuperMemo pioneered the spaced repetition algorithm in 1991 and has spent three decades refining it. The platform offers structured courses across 25 languages and lets you learn from pre-made content or import your own, all managed by the company's original SM algorithm that calculates exactly when you'll forget something and shows it to you just before that moment.

It appeals to learners who want scientific backing behind their study method and who benefit from rigid structure over exploration. The downside is the UI feels dated and the learning curve is steep, but if you're willing to engage with older, less polished software, the algorithm's efficiency is worth it, especially for vocabulary-heavy or technical learning.