Android/iOS: Fing shows you all of the devices on your network, down to the device type and manufacturer for each. Best of all, it puts all of that data in the palm of your hand.
Fing has been around for a while, but it's a quick solution for anyone who wants to take a peek at network activity from their mobile device. Just launch the app to see a list of connected devices, their IP and MAC addresses, customizable device names, and manufacturers. Tap a device to add notes, as or change the name, icon, or location. In addition, Fing keeps a full history of your discovered networks for reference.
If you think you're dealing with something serious, do a more thorough investigation of your network to see exactly what's going on, but Fing is a handy app for day to day use of knowing who you're connected with. It's available for free for iOS and Android.