With this extension, users can log in with their email address or username.
See it live here.
To do this, the attribute name in the input field must be set to name="fields[user-login]". Make sure that the attribute name is set correctly on the pages log in, sign up and forgot password for the input field.
Full example:
<input type="text" name="fields[user-login]" placeholder="Email or username" autocomplete="username" value="" />