However, I've found myself mentioning it to a couple of audiences over the last couple of days as a potential solution to problems they experience, so maybe it's worth mentioning here too.
In brief, what it does is allow you to share a single keyboard and mouse between multiple machines with separate displays, without the need for a KVM.
There are versions available for Windows, Linux and Mac OS, so it's possible to share the keyboard and mouse across different operating systems, as well as different physical machines. This is what I wanted it for, as I thought I might save a bit of impedance switching between machines - and a bit of space on my desk - by getting rid of the abysmal keyboard and mouse attached to the Mac Mini that sits next to my PC. It would be a bonus if I could also drag the mouse through the Mac to my tablet PC.
The first thing to do was to install the program on my main PC - the one to which the keyboard and mouse are physically attached. On this machine, the program will be run in "server" configuration, and information provided as to how the various screens are related to each other. For example for two screens side by side, you might tell Synergy that dragging the mouse off the right of one monitor will take it to the other, while dragging it back to the left will bring it back again. It's also possible to arrange monitors vertically, or in a ring.
You then need to install the software on the other machine to run in "client" configuration, referring to the server across a network connection.
For me, the first client to try was the Mac. Unfortunately, the Mac OS variant of Synergy is the least developed one, and proved a bit awkward to set up for someone unfamiliar with tinkering with Mac OS's underlying gubbins. There are quite good instructions on how to do this, without which I'd have been scuppered, but I eventually gave up simply because it was taking too long to work through them. I did get it working interactively, but didn't have time to waste finishing configuring it to autostart at the two levels recommended.
I also tried installing the client on my tablet, which went a little better, but highlighted a problem that might also have contributed to my Mac frustrations. Due to the limitations of Synergy on MacOS, it was harder to see where things were going wrong.
The problem is simply that Synergy is a bit temperamental about the names used for the server and client machines. They must match exactly on each machine in order for the software to work (reasonably enough, I suppose). As far as I could tell, this extended to having to make sure that case matched exactly and there might also have been some confusion relating to how names resolved.
In the end, the benefit Synergy might have brought me didn't justify the faffing about with configuration that seemed to be necessary. Depending on your requirements, your mileage may vary, so I thought I'd pass this information on.
I originally came across Synergy in the comments to a post by Raymond Chen.