www.political-theory.org

On political theory in general, see the homepage of the ‘Foundations of Political Theory’ section of the American Political Science Association. Among other things, it will give you access to a wide range of key journals.

www.Marxists.org

For Marxist literature.

https://www.ucl.ac.uk/bentham-project/

This, the website of the Bentham Project at University College London, is the best source on Bentham.

www.utilitarian.net/jsmill/

This is a useful site on Mill.

http://plato.stanford.edu/entries/original-position/