I need to explain what this is and – ideally – finish the attached example and use it.
Everyone likes free and if all they have to give away is their email address, many will do that to get access. But that doesn't mean you can give away shit… Put some thought into what you offer. What are you good at? Build around that. For example…
Writing is my strong point so I'm developing short ebooks (you might call them Tiny Books!) that I'll be giving away to drive mailing list signups.
These aren't technically content upgrades, but when one of my students recently showed me Refactoring UI, the illustrations gave me lots of ideas for downloadables.
For example, the following could be downloadable cheat sheets. Perhaps a series of templates that you can refer to (top illustration) or a set of comprehensive colour palettes.
Be creative, what could you give away in exchange for an email address?