email_auth.py

#in your forms
#if you are using  django.contrib.auth.views.login
#pass EmailAuthenticationForm to the authentication_form argument.

class EmailAuthenticationForm(AuthenticationForm):
    """
    Extends the standard django AuthenticationForm, to support 75 character
    usernames. 75 character usernames are needed to support the EmailOrUsername
    auth backend.
    """
    required_css_class = 'required'
    username = forms.CharField(label=_('Username'), max_length=75)
    password = forms.CharField(label=_('Password'), widget=forms.PasswordInput)

    def __init__(self, request=None, *args, **kwargs):
        self.request = request
        self.user_cache = None
        super(AuthenticationForm, self).__init__(*args, **kwargs)



from django.core.validators import email_re

class EmailOrUsernameModelBackend(ModelBackend):
    """Allows a user to login using their email address, and not just their
    username.
    """
    def _lookup_user(self, username):
        try:
            if email_re.search(username):
                user = User.objects.get(email=username.lower())
            else:
                user = User.objects.get(username=username)
        except User.DoesNotExist:
                return None

        return user

    def authenticate(self, username=None, password=None):
        user = self._lookup_user(username)
        if user:
            if user.check_password(password):
                return user

        return None