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Fing is one you might try, from the Playstore.
https://play.google.com/store/apps/details?id=com.overlook.android.fing&hl=en
Why not simply encrypt you WiFi then no one can use ?? If you leave it unsecured then you will have others piggy back off it.
I don't remember if I had sign up. I am using free version. It tells me who is using my network. If there is anything I don't recognize, I know I have a freeloader. Has only happened to me once though.

Thanks Wetbiker. If you broadcast your SSID and don't use security then you always run the chance of picking up a freeloader. It becomes a security issue if they use your connection to surf for kiddie porn or download TV shows, movies or send hate mail to the government. It's best to be secure so things like that don't happen but with WEP being easy to crack and WPA becoming that way it can be hard to keep your WiFi secure. Windows utilities such as AirSnare, Zamzon Wireless, CommView, SurveilStar, and Ethereal do a good job of network monitoring. Depending on your router model open source firmwares such as Tomato and DD-WRT can use built in logging and monitoring utilities.
The Best ones that I know of are Traffic Monitor
https://play.google.com/store/apps/details?id=com.radioopt.widget&hl=en
OS Monitor
https://play.google.com/store/apps/details?id=com.radioopt.widget&hl=en
Along with the others mentioned
I think you misread lol just saying I'm looking for apps that can spy on devices on your wifi network like seeing what someone is browsing on the computer without them knowing
One thing I would like to add since alot of people seem to be reading this thread is the evil that is UPnP. UPnP is a set of networking protcols that permits networked devices, such as personal computers, printers, Internet gateways, WiFi access points and mobile devices to seamlessly discover each other's presence automatically. Sounds good until you realize what a huge security hole this creates. The best advice is to learn what port or ports your software needs to have open and learn how to open those ports in your router rather than have a protocol automatically open that port for you because you never know if it's a legitimate program or a hacker that's knocking on the door.