Socrates at Washington State University Project Gutenberg – e-texts on Socrates:
The Dialogues of Plato (see also Wikipedia articles on Dialogues by Plato)
The writings of Xenophon, such as the Memorablia and Hellenica.
The satirical plays by Aristophanes
The Second Story of Meno; a continuation of Socrates' dialogue with Meno in which the boy proves root 2 is irrational (by an anonymous author)