Thales of Miletus: the grandfather of Classical Philosophical thought. He was interested in why natural materials behaved the way they do and how they change state and form.

Plato (and Socrates): Athenian, 5th century BC Founded the Acandemy. Wrote dialogues featuring his tutor, Socrates, although how much of what he wote was representative of Socrates’ ideas and how much were his own is debated. You should be able to find quotes from both of them very easily. They were interested in the real nature of things, in reason and logic, and in establishing a rational method of enquiry.

Aristotle: Pupil of Plato, Athenian, 4th century BC. Founded the Lyceum school of philosophy. Formed a unified comprehensive philosophical structure for thought that could be applied to natural sciences, logic, morality, aesthetics. His ideas about physics and space were the basics of our cosmology until Gallileo. Also wrote extensively (and very elegantly) on applied philosophy – ethics, morality and beauty and was tutor to Alexander the Great.

Zeno – founder of the Stoic school which became extremely popular in the Roman Republic and remained the hallmark of Roman virtue. Thought that the way to be happy was to retain moral integrity and be more or less indifferent to material comfort, wealth or power. Another good Greek Stoic writer was Crates.

Epicurus – founder of the Epicureans. Taught that the way to lead a good life was to avoid anxiety and lead a life of modest pleasure and tranquility. ‘Modest’ is important here – this is not a wild pleasure-seeking lifestyle but a recognition of the enjoyment only achievable through moderation. Most Epicurean teachings are from the wirtings of Lucretius, a Roman follower of this school.

These are the really ‘big’ ones. Your previous respondant has already given you a complete list if you want some more.

http://en.wikipedia.org/wiki/List_of_ancient_Greek_philosophers