Downloading Duet Display 2.3.3.7 for Mac from the developer's website was possible when we last checked. We cannot confirm if there is a free download of this app available. The software lies within System Tools, more precisely Device Assistants. Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe. This free software for Mac OS X was originally produced by Duet Display.
From the developer:
Duet is the first app that allows you to use your iDevice as an extra display for your Mac using the Lightning or 30-pin cable.
You may want to check out more software for Mac, such as Display Maid, basICColor display or Display Rotation Menu, which might be similar to Duet Display.
Device Assistants
Device Assistants