Wireless Network Watcher is a compact NirSoft tool which can scan your network (wired, as well as wireless, despite the program's name), and report on everything that's currently connected.
For every device and computer it uncovers, Wireless Network Watcher will list its IP address, device name, MAC address, network adapter, and (sometimes) an additional description that may tell you what the device actually is ("Your Router", say).
And the report can be saved for future reference as a plain text, XML, CSV or HTML file.
.As well as being useful for basic inventory purposes, Wireless Network Watcher can also be a handy security tool.
Click Options > Background Scan, then Options > Beep On New Device, and the program will regularly scan your system, looking for new devices (like a neighbour trying to make use of your internet connection, for instance).
And the Advanced Options dialog (press F9) allows you to customise this even further, setting the frequency you need for the background scan, and optionally telling Wireless Network Watcher to launch a particular program whenever a new device is detected.