Advanced IP Scanner
This little free utility is a great way to get a quick list of all the devices connected to your network, listed by IP address, along with information about each. It does a lightning-fast scan of all IP addresses in a range that you specify, then specifies whether a device is present at each address. For each device, it lists the status, the machine name, NetBIOS information, ping information and MAC address.
The program will do more than just scan your network. It also gives you a set of tools that lets you shut down PCs remotely, use the "Wake on LAN" feature for any PC whose network card supports that capability, and connect to remote PCs via Radmin, if it's installed. You can also apply some operations, such as shutting down remote PCs, to a group of computers, not just individual ones.
Advanced Net Tools (ANT)
Here's the Swiss Army Knife of network utilities, and you won't have to pay a penny for it. This freebie puts a whole suite of tools at your fingertips, including ones for conducting ports scans, DNS lookups and pings, and scanning for network shares, checking on routing tables and more.
The security modules are especially useful for quick-and-dirty network scans. There's a network port scanner that can scan all computers on your network and report on their open ports, and a share scanner that reports on all the shared drives on your network.
The information modules are also useful. With them, you can examine your routing table and add and delete entries in it. You can also find out what IP addresses are available to be assigned on your network. Other modules do advanced DNS lookups, let you view all the network adapters connected to computers on the network and add and remove their IP addresses, and more.