CloseTheDoor is a portable security tool which lists all the listening TCP/ UDP ports (to something other than a loopback address) over IPv4/ v6, and shows you their associated program files.
Which, if that's all just jargon to you, means you'll get to see all the programs and Windows services which are waiting for some kind of network or internet connection. This can help you to improve your system security, by identifying and closing listening processes which you don't need (fewer open ports means less chance of you being successfully attacked). You may also spot malware in the shape of some suspicious program which shows up on the list. And it can be interesting just as a way to better understand what's happening on your own PC, the processes you're running and the use they make of your network.
You've probably seen similar programs before - but CloseTheDoor is different. First, because it displays a great of information about each endpoint: the network interface and listening port, the protocol, the service behind it and it's process ID, any associated services, and data relating to the executable file (Company, Product, Description).
If you're not sure how a particular connection is being used, then right-clicking will offer you a great deal of research assistance. So you can open browser windows with common Google searches to find out what a port does, and how to close it; there are similar options for Wikipedia; you can jump directly to GRC.com's port database for more advice; and there's even a link to Sans.org's usage statistics (so you can see if a previously unknown port has suddenly been used a lot in the past few days, a strong indication of malware).
If you decide that a connection isn't essential, then CloseTheDoor can help you deal with that, too. Right-clicking provides an option to terminate the parent process, for instance. Or, if the connection is managed by a service, then the program can stop, disable or even uninstall it in a couple of clicks.
And all this comes in a compact, portable executable (its entire drive footprint is under 500KB), so it's the ideal tool to investigate network security on your own PC and any others you might visit.