I submitted a site as a joke and now I feel bad because there are a lots of interesting sites submitted.

So let me contribute with 2 other sites, maybe not the "coolest" but I think they will be of interest to some fraction of the people here (for those who dont know them)

- http://www.astronautix.com/ A labor of love of just 1 person, almost of 80000 pages! on humanity space programs,you can spend hours and hours reading about obscure soviet prototypes, NASA suits, the Chinese space probes, etc

- https://www.centauri-dreams.org/ A very active (and long) blog about interstellar travel. Despite the fact the subject is now more fiction than science the articles discussed and reviewed are about actual science done by actual scientists (many good guest posts too)