We need a bit more information from you? For what situation, small, or large area, distance from buildings etc.

Eucalyptus gunii - but only if you want a tree that reaches 80 feet.

Plane Trees, Poplars, some Elms, and some Ashes can also grow rapidly, if they get plenty of water. Empress Tree (Paulownia tomentosa) grows quickly in its youth. Note that all of these get huge in time. If you are looking for a small tree that will fill its role quickly, look for some of the faster growing shrubs that can be trained to a tree-like shape.

Beware fast growing trees. They grow and grow and grow....As Tugb says, be careful what you plant or you'll be fighting it forever.