Extend Mac display to Windows 10 is there an app to extend a MacBook Pro early 2016 display (OS X El Capitan 10.11.5) to a Windows 10 laptop? Preferably free, by the way, and also not counting Air Display since the newest version doesn't work on Windows.

The goal is this: just like you could extend the display of a Mac to a monitor using Thunderbolt, I would like to extend it to a Windows 10 screen using some software

So far, I have looked into multiple apps, such as AirDisplay, Splashtop, WIDisplay, Synergy, and ScreenRecycler as mentioned by bwall's answer.
This looks like it will work. It's not free, but I haven't seen anything else that works. It uses a VNC client for the "other" computer, but it says any VNC client should work so you don't have to buy theirs. You only have to buy the Mac program. There are plenty of free VNC clients for Windows. There's a free trial so you can download it and see if it works. 

https://www.screenrecycler.com/ScreenRecycler.html