Question: I want to create a site using a cms, but I want to use my own look and feel. I want to be able to upload downloadable content such as mp3 files with a flash player. I also want users to sign up and login. I want to be able to track and log downloads and uploads done by users. Any suggestions?

Answer: 
There are dozens of different CMS systems and each works slightly different and is geared for a specific use case. Some of the most popular PHP CMS systems are:

[Drupal](http://drupal.org) - One of my favorites. Very powerful and extensible but a large learning curve and for most projects it can be overkill.

[Joomla](http://www.joomla.org/) - Similar to drupal. Easier to use for sure but a little less powerful. For most projects it can be overkill.

[Wordpress](http://wordpress.org) - The premier PHP blog engine. Designed for blogging but can handle most any site type. Very easy to use but you sacrifice some extensibility.

All of these CMS systems have very popular and well documented theme engines and active development communities. I think a choice of CMS has more to do with how you want to use your site rather than your technical needs because at this point there is a large amount of feature parity ( no flaming on this, I know in some cases this is not true but for most mainstream needs they all offer similar features in third-party modules if not built in). For your specific needs you mentioned any of these will probably work. Download them all and try them out. Why not? They're free.

For a full list of PHP CMS try [Wikipedia](http://en.wikipedia.org/wiki/List_of_content_management_systems#PHP_2)
I prefer [Typo3](http://typo3.org/).

Very powerful and great extension repository. For you requirements you dont even need any extension. But it will take a lot of time to get in.