Via reddit: "Go into the App Store and install the app called "Apple Store". Inside the Apple Store app, there is a link to get INKS for free." https://itunes.apple.com/us/app/inks./id1081847121?mt=8
Yup. It's pretty intermittent, but they give away the odd gem (amidst various photo filtering apps) now and then. I think I got Super Sharp that way recently. It's worth checking the app every week or two just in case.
Where is the link? I used to get the free apps before they changed the Store app. Now I can't find the free apps anymore. Wondering if it's not available in all regions.