Optin Monster for popups, subscribe forms, landing pages and tracking
Optin Monster is the best software I have found to create optin forms of all types to make sure your website visitors give you their email address and subscribe to your list so you can contact them with follow-up emails and more information. It includes an easy form builder interface to create your popups and other subscribe forms and has complete tracking including easy split-testing so you can see what is working best and keep improving your results. Also, you can set the forms to be triggered by your visitor's activity, e.g. if they visit certain pages on your site (I have a popup with a free resources guide for left-handed children that pops up on all the pages on my site with “child” in the url).
It can be integrated with lots of other software (e.g. I use it to create nice popups that then add people to my Active Campaign lists together with appropriate tags to tell the AC automations what to do with them). It also has a lot of other very useful functions including the Content Block that gets people to enter their email address to see parts of your content (see my blog post on how I use this here) and their “Exit Intent” popups that sense when people are about to leave your page and give them another chance to register or get your free information.
They have a 14 day money back guarantee so you can try it for yourself and see if it makes you money before you fully commit to it. I subscribe to the Professional version and have increased my new subscribers on the Anything Left-Handed website by over 500% compared to the software I was using before (a lot of it coming from the Content Blocks!).