OpenLayers

OpenLayers is a BSD licensed application JavaScript for displaying interactive "slippy maps". It can load wide variety of common geographic data formats, and has common map controls available of out of the box. But being open source you can add any additional functionality which you might require.

There are large number of websites using OpenLayers to present their map data. Open Street Map uses OpenLayers for its web interface, and many other examples can be found here.

The OpenLayers projects welcomes contributions from new developers and donations through their sponsorship program.