If you're working on some feature that requires you to create many user accounts, here's a tip I find helpful. Use a service like Spaml that creates disposable email accounts just by going to the site.
Drupal allows valid email addresses as usernames. If you can reduce your user creation process to just those fields which accept as valid an email address, you can just go down the form, tab, ctrl+v, tab... and then hit 'Submit'.