Multiplicity

Awesome idea, good realization! Easy to set-up, easy to use. Sharing both controls and audio of the connected devices. Never found anything that can replace it.
I'm praying for devs to make versions for UNIX and Mac OS.

as commercial tool, there is lot of free alternative that has better functionality than multiplicity

Primary functions

• Seamless mode (aka KM): each computer has its own monitor, and you only share keyboard/mouse (feels as if you had a single computer connected to multiple monitors).
• KVM mode: you use the primary computer's monitor, keyboard and mouse. It's like TeamViewer, but in LAN, giving you better latency and image quality, with the downside of not working across the internet (e.g. you can't access your home pc from your work office).

The good

• Great UI: It includes a grid that allows you to easily position the monitors the way you want.
• Copy/paste: It's shared between the computers. Copy text or pictures in computer A, paste it in computer B, and it just works.
• File sharing: You can simply drag-and-drop files between computers to copy them.
• Seamless: In seamless mode, moving between monitors/computers is as simple as moving the mouse to the other monitor, as if you had multiple monitors connected to a single PC. It actually works BETTER than the crappy multi-monitor functionality we have in both Windows and Mac, which blocks your cursor from going to a different monitor in certain parts of the edge if the monitors use different resolutions. Multiplicity is smarter than that.

For a full list of functionalities check their website. These are the ones that I considered useful for my use case and that work well.

The bad

• Fake advertising: I tried Multiplicity KVM Pro for 2 days. Why not longer? Because despite it saying in the website that they offer 30 trial days, that's a lie. Two days later, after I rebooted my computer, it said my trial had expired and I had to pay to continue using it. There was no way around that.
• (Seamless mode) Laggy cursor: When you move to the secondary PC the cursor gets laggy for a few seconds (jumping straight from A to B in one frame when you move it from A to B). Then the lag disappears. I didn't notice this problem in an alternative software I tried. Another problem is the cursor randomly getting laggy (in times other than when switching from primary to secondary PC). This also happens in the alternative software I tried, so I think it's my router.
• Audio sharing: this is useless. The delay is huge and very noticeable.

Overall

It's a great software, mixing both KM and KVM functionalities in LAN. But I DO NOT recommend Multiplicity if all you need is a KM mode, or if you need KVM but don't need it to be on the same LAN. Simply because in these cases there are better free alternatives. I DO recommend Multiplicity if you need KVM in LAN.

• If you just want the KM mode, I really enjoyed Mouse Without Borders. It works better than Multiplicity and it's free.
• If you want the KVM mode, I'd just go for TeamViewer. Sure, the image quality won't be as good, and it requires a decent internet connection. But it's much more powerful and it's free.

Site lists free (trial version) as expiring after 15 days, then you need to pay $29.95 for the standard version.

First one out of all the other alternates that I could get working properly right away after install with no issues so far. The free version does exactly what I wanted to do, use two laptops with one keyboard and mouse between them for a dual display.