I recently found a pretty useful tool called Share Mouse which allows for you to, as the name hints, share your mouse (and your keyboard) across multiple computers.
Just go to their download page and download the version you need, install it and configure it by right-clicking on the new mouse icon in your task bar. You’ll need to have the program installed and running on more than one computer to get it to work.
Especially important setting is the security setting. Make sure to set a password for your computers so that only they are connected (in case a friend on the network decides to install the program as well):
You can also decide which way to place your monitors by clicking on the “Monitor Manager”-button:
This is an excellent program,free for personal use and pretty cheap for professionals!
If you want a version which allows you to copy/drag items between screens/computers then MicroSoft are actually developing something of their own. They call it “Mouse without borders” and can be found and tested here. I’ve actually tossed out “Share mouse” and completely moved over to this!