Horizontal - Much required format if you want to login from a toolbar or header/footer.
Vertical - Useful for logging from the vertical sidebar on left or right.
Popout - This one is a space-saver. It shows just a short stylable 'Login' button. On click, it opens up the actual interface for the users to login.
Bookmark - The login button will stay fixed on the corner of the screen for better accessibility without taking any space on actual web area.
You can easily switch between the three layouts from the Joomla backend Module Parameters. Also these can be furthur styled to suit your template skin.
Stylable to fit on your template : You can skin the login styles to fit your website theme with little knowledge of CSS.
Optimised XHTML and CSS : The markup code is optimised and valid, to improve your ratings on search engines.
Cross Browser Compliance : IE7+, Firefox, Chrome, Opera and Safari
Multi-language support : The module supports Joomla's multi-language features, and you can implement it in regional languages by just copying the language files.
Having also bought their responsive menu I'm very happy to be supporting some great Joomla development.