What is the best mouse and keyboard sharing software?

If you know how to compile code from source, you can follow the documentation to compile the core binaries (server and client) and run them on your machines without a GUI using a hand-crafted configuration. Given how seamlessly and beautifully this works on a LAN, the paid option is more of a convenience that helps contribute to FOSS. The model is similar to Xchat and does not bar you from using the full-featured version.