I've encountered Colorado Backup for tapes coming from Windows systems. It's owned by Hewlett-Packard now. There used to be a limited-life free download.
You can also try OnTrack Data Recovery. I think they can read almost anything. http://www.ontrack.com/datarecovery/servicequote.asp?PartnerID=2337811
Yes, the link has my Partner ID on it, but I don't get a cut...
I do this sort of thing for tapes written by VAX systems running the VAX/VMS (now known as OpenVMS) operating system. Kind of a narrow niche, I know: http://www.stanq.com/conversion.html