OpenDisplay
Free, open-source Mac and iPhone or iPad apps that turn a spare Apple device or Mac into a true second display over USB or WiFi.
OpenDisplay is a free, open-source display system with a Mac sender and receiver apps for iPhone, iPad, and spare Macs. It turns a device you already own into a real additional monitor: macOS treats it as an extended display that can be arranged in System Settings, while mirroring is also available as a separate mode. The sender can stream over a Lightning or USB-C cable through macOS usbmuxd, or over WiFi with Bonjour discovery. The project documents a hardware H.264 pipeline, frame-drop backpressure, Retina or HiDPI output, touch input, two-finger scrolling, and portrait or landscape orientation. The README says that several iPads and iPhones can be used at once, with up to three tested. A separate Receiver app can use a spare Mac running macOS 12 or newer as the display. OpenDisplay is designed for people who want a local, auditable alternative to Apple Sidecar, subscription software, or a hardware dongle. The README describes a direct connection between the devices without an account, server, or telemetry. The project is GPL-3.0 and is not an App Store application because it relies on the private CGVirtualDisplay API; the author notes that macOS updates may affect that part of the system. Audio is currently out of scope. To try it, download the signed and notarized Mac release from the official site, install the receiver app on an iPhone or iPad from the available beta or build it from source, and connect the devices by USB or WiFi. The first run may require Screen Recording and Accessibility permissions on macOS. Building from source requires Xcode 15 or newer, xcodegen, and Apple signing for the device targets. The sender requires macOS 14 or newer; the official receiver supports iOS or iPadOS 16 or newer.