http://curious.astro.cornell.edu/relativity.php

math questions super hard http://www.skytopia.com/project/imath/imath.html#2

general relativity http://www.ncsu.edu/felder-public/kenny/papers/gr1.html

http://www.astronomycafe.net/gravity/gravity.html

http://cosmology.berkeley.edu/Education/BHfaq.html