There are not too many sites devoted to Arthur Koestler on the Web.
There are a few pages of sparse selected quotes (http://www.freedomsnest.com/koestler.html), not really interesting, and some other pages related to the historical meaning of Darkness at noon (http://www.csudh.edu/hux/syllabi/555/index.html) and of Koestler's life (http://www.dickinson.edu/~lieberma/d'aiuto/daiuto404.htm)
Quite different from the other sites are the web of the Koestler Award Trust (http://www.penlex.org.uk/pages/kaindex.html, now quite outdated) and the web of the Koestler Parapsychology Unit at the University of Edimburgh (http://moebius.psy.ed.ac.uk/js_index.html). To learn more about these institutions, visit their Webs or read the related page in Koestler's biography.
A quite interesting page is the one from Authologies (in french): http://authologies.free.fr/koestler.html.
Yet another category of Web sites are the ones that present the full text of his The Thirteenth Tribe, often presented in a quite anti-semitic fashion or more simply in an anti-Israel mood (Hey, but if these jews aren't really jews, why do they say they want to return to Israel?)