The WeoGeo blog highlights the Shaded Relief Archive, a collection of hand-shaded relief maps freely usable and distributable. This is a joint effort from Tom Patterson, creator/manager of the Natural Earth Data website (see this post for more info on the latter site), and Bernhard Jenny (author of the Terrain Bender and Scree Painter tools). While you can generate shaded relief from digital data (using reflectance), hand-shading can often do a better job of highlighting subtleties of terrain detail, as in this comparison of hand-shaded data (left) and digitally-derived data (right) taken from the Shaded Relief Archive comparison page:
Some maps come in georeferenced GeoTiff format (along with shapefiles that define map area and coastlines); others have yet to be georeferenced. Most are at fairly small scales, so they’ll be best for maps covering large areas; an exception is the set of 100+ shaded relief maps for US National Parks.
Note: Some of these shaded relief maps, as well as many others, are also available at the WeoGeo Market site.