there's also https://www.myvocab.info/en

From what I can tell they actually have a bit more robust science behind their algorithm (and a lot less questions to answer)

This one's much better. Shorter, faster, adapts to one's level, gives an out for being unsure, largely doesn't bother with definitions (except the occasional verification challenge), and even mixes in some fake words to ensure you're not BS-ing.