Calculating peaks in DEM using GRASS

Use the grass module r.param.scale with the "param=feature" option. THis creates an output map with each pixel categorized as peak, ridge, channel, plane, etc. Peaks are given category 6 (ridges=5, etc)

A pure pixel based approach could be done with "r.mapcalc" ([..]) but will likely not lead to exciting results. Hence r.param.scale as suggested by Micha or this Addon: r.prominence which calculates the average difference between a central cell and its neighbors. It approximated the terrain 'ruggedness' by looking at average differences in elevation within a given neighborhood. See http://grasswiki.osgeo.org/wiki/AddOns/GRASS_6#r.prominence