Every once in a while I need to uniquely identify a machine. I don't case about its name or properties. I just want something unique to key off and ideally I want its format to be the same on both Windows and Linux.
On Linux you can use content of /etc/machine-id. On pretty much all distributions I've tried this file will be generated on install and will contain UUID. For Windows, one must look in Registry. Under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Cryptography\MachineGuid Windows will keep their unique identifier also in UUID format (albeit with dashes this time).
PS: Note these values can be manually changed by user so you cannot really depend on them for security purposes (e.g. generating licenses). But for machine identification they'll do.