This is a fairly basic mod. It makes several ores - coal, diamond, gold, iron, lapis, redstone, copper, tin (if using those) - spawn in the nether so that you have a reason to go there mine.
These can make you quite rich (with IC2 they even macerate for the double amount of ore per block), but beware, the local inhabitants don't take too kindly to people stealing their resources, and ages of being exposed to the hostile environment of the nether has left these ores somewhat unstable... (They have a chance of exploding when being mined, so if you hear a TNT/creeper detonation noise, run!)
Once you harvest one of these, you can smelt it into the drop you'd get from its surface form by cooking it in a furnace. Nether Iron gets you an iron ore block, Nether Redstone gets you redstone dust, etc. Putting them into an IC2 macerator yields twice the reward!