Remotely-sensed imagery from aircraft and satellites represent one of the fastest-growing sources for raster GIS data. While remote-sensing technology has been around for decades, recent technological advances and legislative changes have led to an dramatic increase in the types of imagery available. In the U.S., due to recent legislative changes and the repeal of the Landsat Commercialization Act, these data can now be obtained at costs well below previous levels. Also, satellite imaging has now been around long enough to allow study of temporal changes on the land surface. For example, early Landsat images beginning in 1972 can now be compared with recent observations, providing a 25+ year record of land-use, vegetation, and urban change.
If you are looking for comprehensive and free data for a project, California hosts a wide range of federal, state and local agencies that readily make their data available at no cost. Many of these datasets can be easily downloaded from the Internet. Most can be found in common GIS and CAD formats such as ArcView Shapefiles, ArcInfo export (.e00) or Microstation (.dgn) files.