TechSini has to be one of the simplest, easiest automatic mockup generators I've used.

ShotBot is a screenshot builder for iOS & Android apps.

Screenpeek is a super simple mockup generator.

Screely instantly turns your screenshot into an image you can share anywhere.

Placeit provides literally thousands of Free iPhone, iPad, iMac and Macbook Mockups in a variety of environments to suit any project.

If you have a cool app and want to show it to your customers, simply upload a screenshot and download high quality images of your app.

Mockuuups provides a drag-and-drop tool for creating beautiful app previews and marketing materials.

Mockupsjar offers some alternative mockups that others don't, including flat design browser mockups which are great to show off examples of your website on social media.

Mockuper is a free online mockup generator that lets you design your products mockups easily and quickly.

A great selection of various mockup environments for iPhone, Apple Watch, Macs, Android, Samsung and many more.

Dunnnk is another good tool to generate website mockup in iPhone, Android, MacBook, iPad and iMac devices with high definition output.

With Dimmy.club you can create mockups for you website in seconds.

Browser Frame provides browser frame mockups in a modern flat design.

AppLaunchPad provides thousands of pre-made mockup templates to select from for free.