Android - How to show wifi password?

From any file manager that allows root access to system files you can go into /data/misc/wifi/wpa_supplicant.conf file or directly search for wpa_supplicant.conf file.

You can open it with any text editor.

If you have Terminal Emulator you can also do this:

cat /data/misc/wifi/wpa_supplicant.conf

You will find all your saved passwords inside that file. Password will be stored as:

network={
    ssid="WiFiName"
    psk="password"
    key_mgmt=WPA-PSK
    priority=1
}

You can also download the app "Wifi Key Recovery" which is a lot easier.