Free software to share mouse and keyboard between linux and windows?

A search on alternativeto (and filtering by Open Source & Linux) returns Barrier, which is forked from Synergy.

There used to be small print on the Synergy website stating that the payment is to download synergy. It's still open source, and you can might be able to build it yourself, but it might need proprietary binaries.

Use Barrier. It is free and open source, was forked from synergy. It is amazing. (Link to GitHub repo)

Installation in newer Ubuntu versions:

Run in a terminal:

sudo apt install barrier

If you prefer snap packages, you can do instead:

sudo snap install barrier

Installation in Ubuntu 18.04:

In ubuntu 18.04 the best way to install seems to first install flatpak:

https://flatpak.org/setup/Ubuntu/

And then barrier using flatpak:

https://flathub.org/apps/details/com.github.debauchee.barrier