LIDAR (light detection and ranging) is an optical remote sensing technology that measures properties of scattered light to find range and other information about a distant target. In GIS and mapping, it's used for a variety of purposes including creating digital elevation models, shaded relief, and capturing the contours of the built environment. LIDAR is also used in atmospheric research and meteorology.
LIDAR data files, due to its high level of detail, are very large and are frequently in large scale (small area) sets. Here are some good sources for LIDAR data:
NCALM LIDAR Distribution Center (UC Berkeley, University of Houston, National Science Foundation)
Open Topography data portal
CLICK (Center for LIDAR Information Coordination and Knowledge) from the USGS
Original post blogged on b2evolution.