I've been playing Elite: Dangerous for over two years now, through the initial release of Wings and Powerplay, and I find these sites absolutely essential. It's ironic actually that they are down this weekend, when an important in-game event is happening around the new Elite novel. I was planning to use these tools not only to refine ship builds for the event, but to find staging posts, bases, outfitting, and so on.
I would call out eddb, Inara, Coriolis and Shipyard as sites that I use daily. Elite simply isn't playable without them. That is to say, of course it's playable, but it would be horribly inefficient and I likely wouldn't bother. In-game tools are weak; I believe it's a Frontier strategy to allow the game to be augmented with third-party tools -- a strategy that I support, I think it makes sense -- but for Frontier to fail to really support those third-party tools makes no sense.
I fully support these tool makers' in their action to call attention to this issue, despite it hampering my in-game play this weekend.