Ninja Forms is the most customizable plugin out there. The core plugin is very easy to use and let’s you create all sorts of forms.
If you want to connect your form to 3rd party services like MailChimp or Stripe they have a ton of add-ons.
I’ve written tutorials & reviews of some of their plugins.
I'm very happy to announce that I've just released a Wysija extension for Ninja Forms! If you haven't heard of Wysija before make sure to check them out – they're an awesome newsletter service that you can manage through your own WordPress site. Now, with this extension you can allow users to automatically subscribe to your Wysija lists when they fill out a form on your website.
Today I presented How to Use Marketing w/o the Shenanigans at WordCamp Milwaukee which focused on getting the most conversions from your contact forms.
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.