Keybr

That intro/tutorial could have fewer than X,000 characters ;) But I like the idea of an AI based typing instructor that keeps learning your challenges and growing. I like the flow you can get into as well, since it's essentially endless.

I actually came across this product quite recently, and in modesty I think that I am already quite a fast paced typer. However, this is actually a great way for younger children and already fast paced, and confident typers to improve their game.