Best free platform for managing free/premium user accounts?

I would like to create a website where users can sign up for accounts, and each account can upgrade to a premium version if the user pays. (Similar to 37signals's products)

Is there a platform with a built-in system for allowing users to make credit card payments to access premium content? A drupal module, perhaps?

 

Although I have not actually used it, the Drupal eCommerce module should work. I have used the LMPaypal module for processing Premium Membership payments, and it works great - but as the name indicates, it only works with Paypal.

 

Havent used drupal but Joomla have similar tools.. Have a look at their extensions..

http://extensions.joomla.org/extensions/e%2dcommerce/paid-access-and-content/1367/details

For others ( but i havent seen them..)

http://extensions.joomla.org/extensions/e%2dcommerce/paid-access-and-content

There is also other community based "plugins" which will be great for you!
Im testing it at the moment - http://www.5ft4.com/index.php?option=com_acctexp&task=subscribe

Good Luck!