I have been a user of keybr.com for a long time, and I didn't really like things like for example only practicing one word at a time or the algorithm trying to force you to type each character at the same speed, that's why I made my own. It has an algorithm that selects words randomly with weights based on how long you take to type each letter, you accuracy with each letter and how common the letter is in English (you should type more-common letters faster!).