An honourable mention to https://typ.ing too. It’s the same kind of tool from the guys behind the moonlander keyboard, ZSA.

I particularly like the spaced repitition at https://www.keybr.com/ and the heatmap of missed keys at https://www.keybr.com/profile.

I find the way that it handles errors to be super annoying—I type fairly quickly and hitting backspace is built into my muscle memory in a way that just seems totally incompatible with that site, even with the "Stop cursor on error" setting unticked.

To be honest I could never get into keybr - typing random words is kind of boring and pointless. I saw much better progress on monkeytype, typingclub and now typequicker

Might be just my preference, but I'd rather train on monkeytype.com. The icon for the space is confusing and the way it consider spelling errors too.