The best free alternative to Spacedesk is Deskreen, which is also Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to Spacedesk and 14 is free so hopefully you can find a suitable replacement. Other interesting free alternatives to Spacedesk are Tab Display, Weylus, QtScrcpy and superDisplay.
Deskreen turns any device with a web browser to a second screen for your computer
It is much faster on browser.
It's just a tool to mirror your screen to any other device via web. It happens that if you have a virtual second monitor (HDMI dummy) you can share that as your second monitor.
Tab Display seamlessly transforms your tablet into a second monitor, allowing you to extend your screen and stream the virtual display directly to your device. This app supports both Android tablets and iPads for use with Windows and Mac environments.
Weylus turns your tablet or smart phone into a graphic tablet/touch screen for your computer.
I like this software. It's lightweight and easy. I try my smartphone and it worked
QtScrcpy connects to Android devices via USB (or via TCP/IP) for display and control. It does NOT require the root privileges.
It mirrors phone to desktop, not desktop to phone
superDisplay turns your Android device into a portable USB display for your Windows 10 PC. Duplicate or extend your screen simply by plugging in your phone or tablet.
LetsView is a free screen mirroring app that allows users to mirror their screen to a larger display in real time, offering superior quality without lag. Easily connect by scanning the QR code!.
Make your iPad/tablet/computer into a secondary monitor on Linux. - GitHub - kbumsik/VirtScreen: Make your iPad/tablet/computer into a secondary monitor on Linux.
This program will let you use two screens on two different computers as if they were connected to the same computer. It even works between different operating systems.
Interact with your small phone on your big display, your good speakers and your keyboard. Share your iPhone/iPad screen in Zoom/Teams.
This program will let you use two screens on two different computers as if they were connected to the same computer.
DFMirage is "cutting edge" video driver mirroring technology for the Windows. It is a driver for virtual video device managed at DDML level of the graphics system that exactly mirrors the drawing operations of one or more physical display devices.
ZoneScreen is able to mirror local desktop to another computer in the network. The main difference is that it also allows extend desktop, not only mirror it. Also, you could use ZoneScreen for Windows Mobile to extend/mirror your desktop to screen of your handheld.
Very old but still OK if you want to mirror the display. There are comments regarding extending the desktop, but it seems to be complicated.
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Can only use to mirror screen - not extend desktop & it has to run through a browser window.