Nala and Neela in the Ramayana are two brothers of the Vanara (monkey family) who help to construct the bridge when Rama had to cross to Lanka.
The two brothers were always up to mischief, stealing things from Rishis (Holy men) and throwing them in the water. One of the Rishis who was constantly losing things put a curse saying that whatever they threw in water would not sink but always float.
When Rama had to cross the oceans to reach Lanka, the help of Nala and Neela was sought. They threw pebbles in the water which did not sink and made a floating bridge.