Free may be hard to come by.... I've never run across any.
I would recommend checking out siteZen by sitemagix.com. It can integrate into cPanel, you can offer your customers a free trial of it, and they have various licensing structures as low as $100 to start or lifetime (including updates) licenses cheaper than some builders single licenses. Best of all, support has been incredible. There is also a forum, though it isn't "heavily" active.
(you know this thing, if you have 100 customers, only one will need them. However, usually newbie will use this. With newbie, if you give them good service, there are no reasons for them to move FOREVER) ROI mann....
One of my prospect said, he stay at Yahoo (expensive plan!) because it has site builder. He will move to mine if I offer it too (of all thing, it's what he request, imagine that!)
We have a system like this on our hosting acounts and they can be a good selling point. However, i do sometimes worry that if you sell a hosting plan to someone and they buy it solely because they want to use this system, and then they are not happy with the results, it could be a problem. I do include this in my hosting packages but I not longer put any emphasis on it in my marketing.