Windows: If you want to create a network plan and need an inventory or just want to see all the devices using your Wi-Fi, Wireless Network Watcher from Nirsoft is a free utility that can help. It scans your network for connected devices and lists their network properties.
The portable program displays a list of each computer or device on the same wireless network, with IP address, device name, MAC address, and network adaptor manufacturer. Besides my router and computers, Wireless Network Watcher found my NAS devices, wireless printer, and iPod touch (though it didn't have a device name for that).
We've previously mentioned ways to find out if someone's stealing your Wi-Fi, including checking the router device lists, but Wireless Network Watcher may be an easier solution. It's also handy just for creating an inventory of all your gear; you can export the devices list to various formats or copy and paste it into other applications.