What are you planning on doing?
There are plenty around but probably the most common would be WordPress, Joomla! and Drupal. Power and learning curve increase from left to right. All of them have active communities and are updated regularly. All of them let you apply your own skins / templates over the top, so you can change the display and layout. You can either buy / download templates, or use a tool like Artisteer to create your own.
Denis