Plato Resources:
Garth Kemmerling on Socrates and Plato (brief and accessible)
Richard Kraut on Plato (A long and fairly difficult article from the Stanford Encyclopedia of Philosophy, but you may find individual sections -- like the one on Socrates -- useful)
Descartes Resources:
Jonathan Bennett's modernized version of the text
Rae Langton's study guide to Descartes' Meditations
Stephen H. Daniel's discussion of Cartesian epistemology and rationalism
Hume Resources:
Jonathan Bennett's modernized version of the text
A study guide by Professor Gerald G. Massey of the University of Pittsburg
Stanford Encyclopedia of Philosophy article on Hume (fairly advanced)
Garth Kemmerling's overview (simpler)
Locke Resources:
Jonathan Bennett's modernized version of the text
Garth Kemmerling's overview of Locke's philosophy of government
Stanford Encyclopedia of Philosophy article on Locke (more advanced)