Well as far as I can tell the last version of nero 6 is 6.6.1.4, but by accident I just found out this isn't quite true... In fact the latest version is 6.6.1.6. It appears to be availbale as an oem download for only one manufacturer, perhaps it deals with a few issues it had with that manufaturer...
You can find it at ftp://ftp.nero.com/oem/FSC/.
Laurentiu