Weebly is offered through WHMCS these days.
Seeing as a lot of hosts use WHMCS and cPanel, this may be counted.
However, SitePad seems to be one of the better ones. Made by the Softaculous team.
Personally, with countless templates available for CMS like Wordpress and Joomla, website builders IMO aren't really needed.
Also, cPanel is making their own. Quite limited for now, but it is expected to improve.