When Kakshi saw Itachi's MS eyes he told Asuma and Kurenai to close their eyes. So he knew about the MS. When Kakashi and co. were looking for Sasuke after the battle against Itachi when Kakashi saw the black flames he knew it was Amaterasu. And finally when he saw Sasuke's Susanoo he said I presume that's Susanoo. How did Kakashi knew about all this after all MS and techs are extremely rare and only a few of the top Uchiha's like Sasuke's father knew about it. Same goes with Danzou how did he knew it was Susanoo when he saw Sasuke's one.
Now that's actually a very good question. I think MS would have been known, at least by some people. If Madara is about 100 years old as I think I read, that's not too long for people to forget about his awesome MS powers. Sure, maybe they weren't teaching too many people about that secret since it requires killing people and the Uchiha don't want people to be murdering their friends to gain power.
So MS is a secret from most people, but it wasn't a secret from everybody. You have to imagine that anybody who knew Madara back when he fought the first hokage would know about the power and would tell stories about it.
Itachi was a wanted criminal, and didn't spend all his time sitting in some dark cave. He had obviously mastered his MS techniques, which means he must have used them. Using something that awesome there will always be witnesses who tell stories. Plus, wasn't it mentioned that Itachi was still secretly giving information back to Konoha about Akatsuki? If so, he could have mentioned some of his own powers.
As to the individual powers, we know that different Uchiha seem to have different powers. So Itachi and Sasuke have Tsukyomi+Amaterasu+Susanoo. Kakashi has different ones (though we have only seen him use Kamui, he might have others like what Sasuke has. We don't know). If Kakashi doesn't have the same powers as Sasuke, I don't know how he would know anything about it. Also, if Madara had different powers from Sasuke (and it seems Madara was surprised about Sasuke's Susanoo, so I don't know if he had that), nobody would have been able to witness those powers in him.
So I agree with you, it's all very strange that so many people seem to know so much about these powers. The only two theories I can think of were that people remembered Madara's powers and taught people about them, or Itachi was giving information about his powers to Konoha. Both of those seem a little bit unlikely. I wonder where this information comes from?
