input-leap software KVM, a way of sharing a single mouse and keyboard across multiple computers. First there was Synergy (which I was an early backer for, got a lifetime membership for an initial donation). When Synergy 2.0 was being developed, which broke everything good about it, there was a fork called Barrier. Now it looks like Barrier has been abandoned and the most prolific contributors have forked into input-leap.

For new users I'd probably recommend input-leap but the original developer of Synergy has abandoned v2.0 and is still making updates to the original so it's still a great piece of software. I use it so I can have my work machine sitting next to my desktop machine and don't have the clutter of multiple keyboards. Just drag the mouse over and now I'm on the work system...