Sir Isaac Newton was most noted for his theories on the Laws of Motion as well as the idea that all bodies exert a gravitational attraction on surrounding bodies (according to legend, Newton found out about gravity when he was sitting under an apple tree and an apple fell on his head - but in reality, Newton was perfectly aware that objects fall, and didn't need to have an apple fall on his head in order to find out; bear in mind that he was among the most intelligent people who have ever lived, and was certainly capable of observing the obvious).
Other major contributions he made to the field of Classical Physics were:
The discovery of spectrum: He allowed sunlight to pass through a prism into a dark chamber onto a white screen. He observed a band of seven colors on the screen. The colors were the same which made up a rainbow viz, violet, indigo, blue, green, yellow, orange and red. He thus conferred that sunlight or white light was made up of seven colors and the rain droplets in the sky acted as a prism which split sunlight to give the impression of a rainbow.
Rectilinear Propagation of Light: Newton also made a contribution towards proving that light travels in a straight line. He made a pin-hole camera for the first time with a hole on one of the sides of a cardboard box, and a tracing paper on the opposite side. One could see an upside down image of the object in front of the hole. This proved that light travels in a straight line.
The equation for the velocity of sound: Newton also made a major contribution to the calculation of velocity of sound. Since this equation was calculated taking into account that sound waves travel in the form of translational waves, there was a slight anomaly in the results. This equation was later corrected by a scientist named Laplace.
It is also worth noting that Newton was a great mathematician, and he helped invent calculus in order to have the necessary mathematical tools with which to state his theories in a precise form. This was revolutionary in itself.