How to get unique machine id?

BSD:

cat /etc/hostid
kenv -q smbios.system.uuid

Linux:
cat /var/lib/dbus/machine-id
cat /etc/machine-id

OS X:
ioreg -rd1 -c IOPlatformExpertDevice | grep IOPlatformUUID

Windows:
reg query HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Cryptography /v MachineGuid