The regular random username generator lets you generate lists of usernames made up of words picked from lists of categories.
However, most of the time people want a username which is personal, and that means something to them. With this tool, you can enter your own word, and then pick from the list of categories to generate related usernames.
The most commonly used parts of the form will be the first 2 fields ("Your Word" and "Category")
However, if you want your username to be e.g up to 15 characters, or exactly 15 characters, you can specify that on the form. You can also choose if you want the free text word you enter to appear at the start or end of the username, or also just let it appear randomly.
Twitter puts a 15 character limit on the length of the username you can use when creating an account with them. Therefore, this tool might give you a helping hand in allowing you to specify how long you want your username to be. Just the username length value from the "Length Limit" pick list (the first of the green fields on the form above).