System account passwords can be found in /etc/shadow. You need root privileges to read the file. The passwords are hashed with SHA. Additional information can be found on the corresponding manpages.
Network or wifi passwords can be found in /etc/NetworkManager/system-connections. There is a file for each connection with its configuration, also you need root privileges to read them but the password isn't encrypted.
Passwords handled by Gnome's password store, the Gnome Keyring, are stored in ~/.gnome2/keyrings. The files are not human readable and should be accessed with Gnome's default password manager Seahorse. On older systems (before precise/12.04) wifi passwords were stored in Gnome Keyring and PINs for Mobile Broadband are still stored there.
