Computing environments such as server rooms often have so many PCs in such close proximity that having a separate display, keyboard, and mouse for each is not only confusing, but a waste of space. You can of course stick with one of each, then attach them as necessary, but that's another time suck. A better solution is a KVM (Keyboard. Video, and Mouse) switch, or if you're cable- and cost-averse, KVM software such as Multiplicity 3.
A physical KVM switch has inputs for a keyboard, mouse, and display and sets of outputs for the same to drive multiple PCs. You run cables to said PCs then toggle between them using switches, a rotary dial, etc. With Multiplicity, you can forego the cables, instead utilizing one PC as the KVM switch (the primary PC in Multiplicity-speak) and your local network as the cabling to transmit signals to and from secondary PCs. Basically, it's remote control software done up to emulate a KVM switch.
Setting up Multiplicity is simple. Install it on each participating PC, defining one as the primary (controlling) and the rest as secondary. When you define a PC as secondary, you're provided with a pass code that you enter on the primary PC to establish contact.
After you've installed Multiplicity on all the PCs, there are two ways you can pass input to the secondary PCs. The first is seamless mode for secondary PCs that have their own display. Simply add the secondary PC to the seamless matrix (3x3 up to 7x7) and when you mouse off the edge of the primary PC's display, mouse and keyboard control pass automatically to the secondary PC situated in the matrix that borders. It's exactly like a multiple-monitor setup where the mouse travels from one screen to the next, except that each screen belongs to a different PC.
Scenarios where seamless mode is useful include environments where separate PCs are tracking different stock markets or displaying separate surveillance cameras, as well as multiple displays monitoring the health and activity of the PCs they're attached to.
The second method is full KVM mode, which is exactly like using remote control software. Right-click on the Multiplicity icon in the system tray, select KVM mode, and select the PC you want to control from the list. You can KVM a secondary 'PC' full-screen (default) or displayed in a window.
Multiplicity will also pipe audio from a secondary PC to the primary PC or vice versa, which can be handy. Version 3 also adds the ability to drag and drop files between the PCs in the matrix. That's all nice, but I do have a couple of gripes.
Multiplicity 3 is not entirely intuitive to the uninitiated, yet the HTML docs are available only online. Sure the Internet is ubiquitous in this day and age, but I've run across many a server locked out of the Internet, or on which you must continually confirm pages as safe. In other words, Edgerunner should bundle the rather small help docs with the program for local access.
I'm also at a loss as to why you can't invoke KVM mode for a PC by double-clicking on its entry in the settings dialog or send the focus directly to a PC by double-clicking on its entry in the seamless matrix. True, it's a settings dialog, but...
Multiplicity was developed by Stardock software, but is now being sold by Edgerunner, a company it backs financially according to the press release. Edgerunner refers to Multiplicity as "disruptive" software. By that I assume they mean upsetting the status quo, as in hardware KVM, but whatever the meaning, thankfully, you don't have to hassle with an account to buy its software as you do with Stardock. Simply pay the money and activate over the Internet.
Multiplicity 3 comes in three flavors: the $20 KM, $40 KVM, and $80 KVM Pro. KM provides only seamless mode with two PCs, KVM allows seamless mode with up to nine PCs and full KVM mode for two, while KVM Pro supports nine PCs in both seamless and KVM mode.
Multiplicity is obviously a niche product. The average user can get away with remote desktop or VNC. But if you've the need, Multiplicity is a handy way to cut down on cable clutter and control PCs that might not be close enough to reach easily from a KVM switch. All in all, a very handy program under the right circumstances.