Not exactly a Form Builder, but solves the same purpose. Using Tars PPC Chatbots, you can create a custom conversation workflow, which gets available to your users as a chatbot. it asks the users all the relevant question following your workflow and each user response is collected as the form data entry.
The best part is, if the user drops the conversation anywhere in the middle fo the conversation, you still get all the data they have given uptil then as a partial lead and you get an email notification with that data.
You can also go to the Dashboard to download all the data as a CSV/Excel File.
PS: I am one of the makers of this Product. So totally biased. Give it a shot. You might like it.