I believe the mod you may be looking for is MobMap, it does this and allows you to browse NPC vendor items, mark up certain mobs on your minimap/world map, and search for mobs that drop specific items / quest drops etc. Like Lightheaded it integrates into your quest log, and also your world map.
Be warned though, it is a real memory hog. Can be found at
http://www.mobmap.de/.
Hope this helps
