This Plugin allows your user to delete his/her own account. If you want, you can also display somehow painfull thank-you message on his resignation.
WordPress can allow your user to register by himself, but has no function to self delete.
This plugin allow your user to quit your blog and delete his own account.
You can also create "Resgin Page" which displays message to your customer.
"Are you sure to delete your account? We miss you."