A tech columnist once wrote that we all have Ferraris idling in our driveways. He meant by his analogy that we all have very powerful computers that we're not using to their potential. I agreed with him. It was certainly true for me. Then I started using a computer for my job, and I found the one I had wasn't enough. I needed more computing power; more screen real estate. The simple solution was to add a second computer, but controlling both with the same mouse and keyboard has been been anything but.
This is free open source software. It shares the mouse and keyboard well. It also does a fine job sharing the clipboard, but doesn't do file transfers.
Unfortunately it has a bug when the mouse is on computer B and computer A has a program demand attention. The mouse can then only be used in the task bar of computer A. You can simply close, and reopen each time, but I have a program that I need to run that often steals attention. It does not work for me.
This is free software, but they do ask for donations. It's kind of a pain to set up. Once you do, it does a great job sharing the mouse and keyboard. It doesn't do file transfers, but it does share the clipboard.
Unfortunately I found the clipboard sharing to often fail. A reboot fixes, but too many reboots later I moved on.
J5 Create Wormhole
This is a $15 cable that has hardware built in at both ends that offers to install its necessary companion software. It shares the mouse, keyboard, clipboard, and transfers files. What makes this different is that it does it all over the usb cable, not your network. This is well worth $15, if it works, and it did work fine with the two Windows 7 computers I tried it on.
Unfortunately it has two issues. First, the program it needs has no option to install. It runs from the device every time, and asks UAC every time. I wrote the company, and they gave me a simple solution... turn off UAC.
The other problem was a speed issues on the secondary computer, but I think that was a conflict between two mouse drivers because I was using two different versions of Windows. It is supposed to be cross platform. I would have chalked that up to a unique personal tech problem instead of a failure of their product, except their tech support never even bothered to respond when I asked.
Mouse Without Borders
This is a free Microsoft product that runs in the .net framework. It also shares the mouse, keyboard, and clipboard. It does transfer files, but there's some limit on size. Setup was as simple as typing two words on computer B, that were displayed on computer A.
And I haven't found a down side yet. I highly recommend.