The National Map: USGS's Digital Initiative

Did you know that virtually every California 7.5' USGS topographical map is available for download? These maps are available in the GEOpdf format, which is a geo-referenced format that allows you to mark-up the map, and make measurements. Moreover, there is a free Adobe Acrobat plug-in that allows you to interact with these maps. In addition, the plug-in interacts with Google Maps, so that  a single click on the pdf will take you to that position in Google Maps.

MapTech and Topo! are still better tools for use during missions, but these maps with the plug-in functionality are still useful for planning and for those who don't have other programs available. If you have access to large format printer or plotter, then you can even make your own map copies.

Currently the plug-in is only available for the Windows platform.