Signup is not free, the account is not free.
From the Formstack webpage at http://www.formstack.com/wordpress.html
"All you need is a Formstack account and the plugin. Signup for free at Formstack"
Clicking the "Signup for free" link brings users to a page with 4 pricing options, costing up to $159/month. You have to pay to get the account; you need a paid account to download the plugin. Ergo: Not free.
I don't trust a company that acts counter to its claims.
This is especially disconcerting since the company collects/stores each form and all data submitted through the forms.