Twitter Bootstrap Alerts
Alerts from Twitter Bootstrap 2.0 used to style error, success, warning and info messages. With the release of version 2.0, it seems like the old one liner messages styles (which was quite nice) have been removed. The good thing about the new version is that, they've simplified the base class: .alert instead of .alert-message and also <p> is not required by default, just the outer <div>. Messages that needs more text and padding will need the .alert-block class now.
You can apply some jQuery, pure JS or the plugin that they provide to make the close button functional.