http://www.iep.utm.edu/republic/  (Internet Encyclopedia of Philosophy)

http://homepages.gac.edu/~arosenth/160/Outline_of_The_Republic.pdf

http://people.wku.edu/jan.garrett/outlplat.htm

Two excellent pieces on the REPUBLIC in the SEP:

http://plato.stanford.edu/entries/plato-metaphysics/

http://plato.stanford.edu/entries/plato-ethics-politics/