Windows: Whether you're troubleshooting poor performance on your own wireless network, or you're trying to figure out how overloaded the hotel Wi-Fi is where you're staying, Who Is On My Wi-Fi is a free Windows utility that can show you exactly what the name implies.
Once installed, Who Is On MY Wi-Fi lets you set an IP range that you want the app to scan, or you can just let it loose to scan your entire home network to see what devices are connected. You'll come back with IP addresses, MAC addresses, computer names (if they're broadcasting), connection status, and more. Perhaps what's most entertaining about the app is that it doesn't require you to have administrative rights or access to the network to run. Even if you're just a client on the network, you can fire it up and see who else is using it. Once you return a list of connected computers, you can make notes about which is which, mark off which devices are yours, and dig a little deeper into devices that aren't familiar.
Granted, if you're using your own home network and want to find out if someone's freeloading or has broken your Wi-Fi password, you can just log in to your router and check. If you're out somewhere and using someone else's Wi-Fi, or you're troubleshooting another network (or you're just nosy), this app can come in handy. Before you go firing it up on your company's office Wi-Fi though, keep in mind that it's essentially port scanning, which may or may not set off a few alarms with your IT department. You don't want to have to explain to your normally-friendly network security engineer why you've been mapping the company Wi-Fi with an unauthorized tool.
Who's On My Wi-Fi is free, and the video above shows you how it works. It requires Microsoft. NET framework to run, and you can grab it at the link below.