module for ajax username checker
A quick search with &quot;drupal ajax username checker&quot; points me to the Username originality AJAX check module. There is a dev version available for 7.
Some more details about this module (from its project page):

This very simple module allows visitors to check username originality quickly using AJAX request during registration (completing registration form). This makes registration process more convenient as you don't need to submit your registration form to find out whether this username is already taken or not. This module is sensible to standard Drupal username validation as well as Access Rules validation.
username_check module provides two ways of user name checking:

Manual (5.x and 6.x) — a &quot;Check availability&quot; button appears on the form and a visitor can press it to verify whether his name is available or not.
Automatic — check is performed automatically when you finish typing and on leaving username field.