[samuellb]

If you use NetworkManager, you can right click its icon, choose Edit Connections. Then select the desired network, and choose "WiFi Security" and then "Show password".

Connection configuration, including plain text passwords, are stored in /etc/NetworkManager/system-connections/ for connections that are available to all users. They are readable by root only, obviously. Otherwise the password is stored in your keyring (e.g. Gnome Keyring under Gnome).

[AvImd]

For the lazy ones:

  sudo grep psk= /etc/NetworkManager/system-connections/*

[keithpeter]

wicd GUI network manager (I use this with Debian/XFCE) basically same as first paragraph above - click on try icon, wait for wicd to finish scanning, find your current connection, click 'properties' find the 'key' field and click the little 'show' box. There is your password.

From a terminal as root you can

    cat /etc/wicd/wireless-settings.conf

and find the password in the file.