You're going to be able to steal the passwords of those users connected to the same WiFi network as you. Especially if you meet two requirements: having a rooted smartphone and having downloaded FaceNiff.
The app to steal passwords
It's an application that can intercept the profiles on an open web session on the same WiFi network that you're connected to. It works exclusively on the native Android browser, therefore, forget about using it with Firefox, Chrome or any other one. In other words, if somebody logs into Facebook, you can steal their credentials. This application has obviously only been developed for educational purposes... or that's at least what its developers say.
These are the services compatible with this app:
And don't forget that stealing passwords and snooping on other people's accounts is not cool... so how you use this app is under your own responsibility.