I just discovered this. You just type stuff and try to get better. You can either practice by yourself to improve your time, or race against friends and other users to see who's the fastest typer. You don't have to sign up. You can log in as a guest. Now I stink, but I've always wanted to get better and this seems fun. It also points out when you make an error and you have to correct it before you can move on.
57 WPM, I've gotten a little faster over the years. I like that it makes you correct the mistake, that's how I naturally type anyway. I don't like the ones that count mistakes.