What do you do when you need to add another form to your project?
There is always a market for a better form builder. Forms for Indies - I think this might work. Something specific around coming soon pages, pre-sales, user surveys, etc. It is a different use case, and a niche.
@shakedko, I used this, but their forms simply don't work. They disabled backend or something, but some other makers reported they were not getting answers as well.
To answer your question: I just use Rails' built-in form helpers. It's pretty straight forward building forms like that.
If I need something standalone (e.g. for a product that doesn't have a Rails codebase yet) I use Typeform which works okay for me.
You might want to have a look at https://formkeep.com if you want something in-between Typeform and self-coded.