Just a few weeks ago I saw a review by Pippin about Ninja Forms and I was immediately impressed. It is intuitive, well documented, & open source; all without a yearly fee. At the same time I was trying to get the most out of my contact forms and I was adding hidden fields that allow me to capture additional information (like geolocation, browser, operating system, etc) about the user without bothering them.
So, in the interest of getting the most out of every opportunity presented to me I took the plunge and developed User Analytics for Ninja Forms.