My development of a general theory of proportions resolved the Pythagorean crisis regarding the existence of incommensurable lengths, such as sqrt(2).
Some consider me to be the "most brilliant mathmatician before Archimedes," a claim that makes me blush with embarassment.
I was born in Cnidos on the Black Sea, and set out at age 23 to learn geometry from Archytas in Tarentum and philosophy from Plato in Athens.
Later, I earned a living by founding schools in Cyzicus and Athens (the latter was a strong competitor of Plato's Academy).
As to other math ideas, I contributed many new ideas to the study of the golden section and invented the mathematical method of exhaustion.
Eudoxus (408 - 355 B.C.)