Socrates was written about by a number of his contemporaries, most famously by his students Plato and Xenophon. Aristotle, Plato's student, also provides some information about Socrates, and other authors wrote Socratic Dialogues which have unfortunately been lost. Aristotle claims that the first of these, pre-dating Plato, was Alexamenus of Teos. During Socrates' own lifetime, he was portrayed in a very unflattering manner by Aristophanes in his play Clouds.

The longer version of the short answer given by Phantomn below, therefore, is Plato, Xenophon, Aristotle, and Aristophanes, alongside others whose works have not survived to the present day.