Useful if you’ve forgotten the root password, or managed to make a typo when setting up Nessus.  This works with Nessus 5 (haven’t tested with other versions):

rm <nessus_install_dir>/var/nessus/users/<admin_username>/auth/hash

echo “newpassword” > <nessus_install_dir>/var/nessus/users/<admin_username>/auth/password

My install directory on Ubuntu 10.04 was /opt/nessus