The Ore Dowsing mod provides items called Dowsing Rods which can help find ores and other blocks. When you right-click with a Dowsing Rod it highlights ores which it finds in a cube around you (sized from 9x9x9 to 17x17x17 depending on the tier). Which ores it highlights is also dependent on the tier.
The highest-tier rod (diamond) can also be set to target a specific ore of your choice, or even non-ores. This can be used for various cheaty and non-cheaty tasks, such as finding chests in a mine or stronghold, or highlighting a pipe system in your base.
Wood Dowsing Rod revealing some iron ore:
Diamond Dowsing Rod revealing coal, iron, redstone, lapis, gold, and diamond ores:
Minecraft 1.8 requirements:
Minecraft Forge, built/tested with version 1.8-184.108.40.2065
Minecraft 1.7.10 requirements:
Minecraft Forge, built/tested with version 1.7.10-10.13.4.1448
You can already detect modded ores, anything ore-like registered in the ore dictionary should work.
I also set up colors for a bunch of common ores (certus, copper, lead, mithril, nickel (ferrous), osmium, platinum (shiny), silver, tin, and all the Thaumcraft ores). I thought about tackling Metallurgy but I was daunted.
I recommend nerfing' it a bit, maybe reducing durability and/or range possibly? And to be honest, it would be nice to make it so that the ores weren't outlined with color, that way it wouldn't be as abusive. One last thing, is it possible to make CoFH core optional for 1.7.10, not everyone wants to use RF to power it. But other than those things, its great!
I've uploaded the 1.7.10 part of version 0.7. This includes a config file which can control almost everything about the mod, and it removes the dependency on CoFHLib. The Minecraft 1.8 release for the 0.7 version is in the works.