There's Landsat data, that requires a bit of processing, and some low-res Blue Marble GeoTIFFs. For very small subsets, we're also really interested in kite mapping by grassrootsmapping: http://grassrootsmapping.org

If you are thinking of high detail imagery common for major cities in the US then most of that is from flights and not from satellites. Some states have Aerial imagery as a state wide coverage available to the public.

I know that Montana has these layers and many others available on the NRIS.mt.gov website as actual GIS data services which are free for people to use.

Yahoo and Bing (Microsoft) allow OpenStreetMap volunteers to trace from their aerial/satilite imagery. You cant use the images yourself, but they can be used to make a good open source map.

Notably Google do not allow this.