At a forum I was registering to, there was a button directly to the right of the "Username:" field that said "Check Username". Upon clicking it, another page opened, and I got the following message: "The username '[insert username here]' is ready for registeration, please go on." If the username was already taken, another page opened and I got the following message: "Sorry, the username '[insert username here]' is already in use, please choose another one."
Does such a feature exist? I have interest in using it.