A talk by Clement Greenberg in 1980 on the autonomy of art

http://newcrit.art.wmich.edu/cg/Autonomies.html

Three essays by Greenberg available on line

http://www.dangpow.com/~sam/greenberg/

Wikipedia, a free-content encyclopedia (fairly good entries on theoretical, cultural topics)

http://en.wikipedia.org/wiki/Main_Page

Jahsonic.com A Vocabulary of Culture. (Idiosyncratic but fun)

http://www.jahsonic.com/

Francis Alys "The Thief" (on the topic of "windows")

http://www.diacenter.org/alys/

Lev Manovich

http://www.manovich.net/