I've always liked TCPView from SysInternals as a graphical way of viewing open network connections by process and port on a Windows machine.

Now I have a new reason to like it. I needed to do some testing of how our application handles network failures. I was figuring I'd have to pull network cables or other painful procedures to simulate this. However, I found that TCPView has an option to let you close a network connection through the right-click menu: