We have selected Hubblesite as the world's very best website on Black Holes.
The Hubblesite website is published by the Office of Public Outreach of the Space Telescope Science Institute. The Institute, based in Baltimore, Maryland, USA, undertakes the planning, scheduling, and outreach activities of the Hubble Space Telescope. It also provides data archiving and distribution services for the Hubble Space Telescope.
The Hubblesite website contains a substantial section on Black Holes. These are objects in space, predicted by the theory of general relativity, with a gravitational field so powerful that even light cannot escape its gravitational pull.
The Hubblesite website on black holes is organized into two main sections: Journey to a Black Hole, and Black Hole Encyclopedia. The Journey to a Black Hole is an interactive experience, helping users to see the sky in terms of X-Rays and Radio Waves as well as Visible Light. A virtual telescope can be dragged across the sky, using a cursor, to see objects in each of these three ways. The Black Hole Encyclopedia contains illustrated articles on topics including: What is a Black Hole? How Big is a Black Hole? and Do Black Holes Live Forever?.
The Hubblesite website is at this web address: http://hubblesite.org/explore_astronomy/black_holes/