Our clients frequently require a very simple OpenStreetMap basemap on which to overlay their own data, for example cycle or hiking routes.
The standard OpenStreetMap map style comes packed with a lot of detail. It covers many thematic areas and gives good feedback to mappers working on improving the map. That richness in detail, however, leads to a very colourful map that is unsuitable as a background map. You simply haven't got enough colours left to highlight what is important, and the OSM detail drowns out whatever you want to display on top.
We have created a simple base map style for these kinds of applications, using only a minimum of colours. For example, we only distinguish forest and other planted areas, and all streets are shown in grey (whereas they are colour-coded by type on the standard OSM map). The style is often used for outdoor applications so we've added hillshading and unobtrusive height contours. Zoom in a little and you'll see buildings - but that's about all there is on this style.
The map style has been designed for the Mapnik rendering engine. For altitude information, we have licensed an improved SRTM data set from CGIAR. The contour lines are kept as vectors in a PostGIS database, and the hillshading comes from raster tiles preprocessed with GDAL.