Free Web Hosts that support All Drupal functionality

DrupalGardens doesn't have "ads" per se, it just has a small image at the bottom that says "Built with DrupalGardens" or something like that. It's a block so you can put it anywhere on the page you like and even hide on certain special pages if you like. You can also pay a reasonable fee to have it removed and or apply your own custom domain to it, e.g. example.com instead of example.drupalgardens.com.

However DrupalGardens is not a hosting service that lets you upload whatever you want; It is a hosted Drupal service. That means you can't modify or add or change code or modules or themes, but you also don't have to manage any services or pay hosting fees or have a sysadmin. There are advantages and disadvantages, but on the whole I find it highly recommendable, as long as you don't need to create custom modules, themes or apply patches.

Further, Gardens is by Acquia, so there is little chance of it disappearing, becoming obsolete or unsupported.

In many ways, DrupalGardens is to Drupal, what Wordpress.com is to Wordpress.