As maps get busier, marker clustering is likely to be needed. Marker clustering is a technique by which several points of interest can be represented by a single icon when they’re close to one another.
Mika Tuupola wrote a PHP library that divides the map into a given number of cells and represents all the markers present in the same cell by a single icon. This icon shows the number of markers it symbolizes.
He also wrote an excellent post explaining how marker clustering works.