I'm not sure how many of you have heard of ITS. It stands for the Incompatible Timeshare System. This was developed at MIT, by hackers, for hackers. Before UNIX, it was the preferred OS of the hacker. The quirk with ITS is that it has no security. None whatsoever. Usernames exist only for the convenience of being able to identify each other. In many ways, Wikipedia is application of these principles. Stark proof that signal can, in fact, outpace noise.
I have been of late hunting high and low for a usable ITS image. I found a distribution that is far more complete than the one I had been using, but everyone linked to ftp.its.os.org for it. That site is now down, and it looked like the image had never been mirrored. Astoundingly, this is where the FreeBSD ports tree comes in. One pdp10 emulator of note is klh10. This was ported to FreeBSD, and entered in to the ports tree. Somewhere along the line, the maintainer(Turing bless him) included the image along with the port sources! This is, to date, the only place I have been able to find with a mirror of the file. I plan in short order to organize hosting, so there is perhaps more than one place hosting it now.