These programs that allow you to share keyboard and mouse input across multiple computers over the LAN, without requiring any additional hardware. Input sharing fills a certain niche that might not be covered entirely by a KVM switch or remote desktop.
I’ve only had experience with Synergy, but I hope to try out the other ones when I have some time and a use case to test them out.
When everything is working correctly, Synergy lets you control Windows / Mac / Linux machines from a single system over your LAN. Sharing input across different operating systems is not an easy task though, and sometimes problems happen that require restarting the service and/or entire systems.
I think overall stability has gotten better since I last tried it a few years ago. There are sometimes conflicts with mouse input in games — here is a Github thread on some known issues. Behavior is more predictable in a serious work environment.
Synergy’s licensing and status is a little confusing. There used to be a free and a premium/pro version, but now, my understanding is that it’s technically open-source and free, but Windows binaries are behind a paywall. You can build from source however — I think?
Note: My Synergy version is 1.7.6.
As said above, I have not tried the programs below, but I keep track of them in case I have a use case for Windows-only input sharing.
Mouse Without Borders
Microsoft’s Mouse Without Borders provides input-sharing across up to four Windows systems.
Input Director seems to get favorable comparisons against Synergy if you only need to control Windows machines. Will add any thoughts if I have a use case for it. Free for non-commercial use.
Multiplicity is Windows-only and paid software.
ShareMouse works with Windows and Mac. There are free and paid versions.