I have just been trying out the new GRASS Map Swipe tool For now it is available in GRASS 7.0 but I understand it will be backported to GRASS 6 after some testing.
The tool allows GRASS users to interactively compare two raster maps of the same area. This is done by a horizontal or vertical split without duplication (a contiguous image of the two layers) or with duplication (i.e., of both layers the same region is shown).
Below a short video where I am using the tool to compare the MODIS 2001 and MODIS 2009 landcover maps (MCD12Q1).
A pretty neat tool, which makes it really easy to quickly compare maps, e.g., maps from different time periods or different model outcomes.