A common grievance for security professionals dealing with Nessus reports is the organization of the report by host or IP address. This makes it difficult for organizing findings by type of vulnerability. This script is a little more complicated than "nsfix", but probably more useful. Enjoy.
(I reserve the right to be somewhat embarrassed if the Nessus experts come out of the woodwork with an option to do this, too, from the Nessus GUI..)