All the eternal elements are very useful for crafting, but I believe there are three that stand out the most; shadow, earth, and fire. Most notably, they are the ingredients to titansteel bars, which are used extensively in level 80 armor pieces. The first two are however a lot more common than fire because you repeatedly receive them from ore mining and various other actions. Eternal Fire is different, for myself have only found two spots that have a decent drop rate.
Both cases involve simply killing fiery revenants, so it is all fairly straight forward. The first location is a small canyon, east of Dun Niffelem, which is the same place you will complete the Hot and Cold quest, for the Sons of Hodir faction. The downside of this spot is that the drop rate is a bit irritating and to get just Crystallized Fire for one Eternal Fire can sometimes be frustrating. However, the revenants spawn reasonably fast.
Second spot is in Wintergrasp, at the very south-east corner, beyond the Flamewatch Tower. The drop rate is much better here than it is in the Storm Peaks, though however, the respawn time is less user-friendly. Usually I simply run it through once and leave again, with enough pieces for one full piece of Eternal Fire.
For the relics of ulduar, the hunt is a lot simpler. They have a chance to drop from nearly every creature you kill in the Storm Peaks, with various rates of course. The best place I have found, is a small cave just north-west from Frosthold. The cave is full of iron dwarves, all with a quick respawn timer and high drop rate for the relics. At level 80 you can pretty much plow through the cave nonstop and in just half an hour, pick few dozen relics.
Unlike Eternal Fire, the relics are not used used for crafting, but to gain reputation with the Sons of Hodir faction. The faction is a bane to many people, primarily because they do not have a tabard that you can wear in heroics, but instead you have to simply acquire their reputation through tedious daily quests. So the ability to farm hundreds of relics, much like runecloth for the racial factions, is a pretty important shortcut.