This one isn't free -- it's shareware, so it's free to try but costs US$40 if you decide to keep it. It scans your network, identifies every device on it -- including computers, servers, printers and more -- and gives vital information about each. For every device, it tries to identify the IP address, the DNS name, the Ethernet address, the server software, the manufacturer of its network card, the user who's currently logged on, ping response and more, such as open ports. For each device, it also includes useful weblinks, such as a link to the network card manufacturer to get patches and firmware updates.
It's a pretty bare-bones program, and its best features aren't easy to access. For example, it's tough to know, at first, how you can scan a network range. To do it, you need to choose Scan Range from the File menu and fill in the form. Make sure you click "Log results to file" to create a log file so you always can refer back to the results. You can also use the program's Cache Manager tool to see information about all the devices on your network.
Be aware that this program takes its time going about its work, so if you have a lot of devices to scan, be prepared to wait. You'll be able to use NetPeek for free for 30 days or 500 scans, whichever comes first. After that, you'll have to pony up for the registration fee.