Mouse Without Borders lets you control different computers seamlessly across a range of monitors sitting next to each other using the same mouse and keyboard. Think of it as software KVM on steroids, similar to programs like Synergy (open-source and multi-platform) that achieve the same result also by relying on your network connection. In this case, Mouse Without Borders is Windows-only but extends to other useful functionalities and comes in as a tiny 1.1mb download straight from Microsoft.
The program was coded by Truong Do, a developer for Microsoft Dynamics, who thought a tool of this kind would be useful for his daily work routine. If you have multiple monitors sitting on your desk hooked up to more than one PC, Mouse Without Borders will let you move your mouse between screens. The software also allows you to lock or login to all PCs at once, drag and drop files between PCs, copy and paste text, and as an added bonus, customize the Windows logo screen with a custom set of pictures or Bing's daily image. Here's a video that illustrates the concept:
The release of this freebie reminds me of the Windows Powertoys and the good old days of tinkering with Windows 95 and XP. This is not exactly the same but Mouse Without Borders is the result of a Microsoft Garage project where engineers spend free time working on side projects, and as far as I recall that's also how the unsupported but widely used and acclaimed Powertoys came into existence.