TOGoS's Minecraft Map Renderer
java -jar TMCMR.jar <region-dir> -o <output-dir> [-f] -f ; force re-render even when images are newer than regions
This will generate a PNG file for each region into the output directory along with tiles.html, which references all tiles in an HTML table so you can view the entire map.
If you run it again for the same regions and outputs, only regions that have been modified since their corresponding PNG files were generated (according to the filesystem timestamp) will be re-rendered.
See also: TMCMG (for generating entirely new terrain).