There is SpiceOpus, which is a spice3 implementation. You can find more information and download it (I think there is a node count limit) at **broken link removed**. There is no link with Cadence.
(Cadence simulator is Spectre. Also Cadence have a script program called Ocean.)
Regards
Opus was used long back .. before the icfb came in Cadence world. So Opus mostly stands Cadence software and more specifically icfb( ic front and backend) . which includes all virtuoso tools.