A very common thing you see on the web is for sites to offer free downloads to visitors after getting them to sign up for an email newsletter. Perhaps it might be a whitepaper, or a free ebook.
From a marketing perspective, the goal is to offer something of enough value for free that people are willing to give you their email address. You are then able to do follow up marketing.
The challenge from the site design is that you want to make sure you collect a real email before you hand over the free download, you don't want people to just sign up with email@example.com.
The are two basic ways to achieve this. For both you need to use a 3rd party email newsletter service such as iContact.