On versions prior to Plesk 10, you can find the Plesk admin password in:
/etc/psa/.psa.shadow
For Plesk 10 and beyond, use this command instead:
/usr/local/psa/bin/admin --show-password
Even more interesting is the MySQL admin password. For all versions of Plesk, the MySQL admin password is the string contained in /etc/psa/.psa.shadow. In Plesk 10, the password is the hashed value stored in this file.
