Terraria – find Hellforges in the Dungeon

You can go exploring some corrupted/crimson areas (randomly picked at world generation) to destroy the shadow orbs/crimson heart waiting for you at the bottom of chasms.

Each shadow orb/crimson heart destroyed will provide you some loot, and make a meteorite land on the world. Once the meteorite has fallen, go find the site, and mine the meteorite ore. This will allow you to craft some meteorite gear and weapons.

Once you've destroyed three shadow orbs/crimson heart, it will make the Eater of Worlds/Brain of Cthulhu spawn, allowing you to fight two different bosses.

Note: The shadow orbs are protected behind some ebonstone/crimstone, which you will not be able to mine with your current pickaxe. Just buy some dynamite from the Demolitionist and use it to access the shadow orb. Or you can buy purification powder from the Dryad, which turns the ebonstone/crimstone into the easily destroyable standard stone.

You shouldn't use it as a building material because the presence of a lot of Ebonstone causes the sky and music transition seen in the Corruption, and allows Corrupted enemies to spawn.

What you could do with it:

• Place it in your inventory's trash slot to get rid of it; use Shift-Click for maximum efficiency.
• Place it, then purify it to get more Stone, if you happen to be short on that (which will never happen because half the map is Stone).
• Store it on the assumption that it does something useful at some point in the future.
• Use it to lock a passage to players who don't have a Nightmare Pickaxe or higher yet. Then again, other materials, such as Obsidian or Hellstone, would be better suited for such purposes.
• Use it to build a Corrupted enemy grinder for infinite Rotten Chunks.

As of patch 1.1, there are also a few more "real" uses:

• You can combine it with Demonite Ore to make Demonite Bricks, which are a feasible building material.
• After unlocking hardmode, the Corruption can spread via Stone Blocks as well, so you can get an Underground Corruption going by planting a few Ebonstone Blocks, then waiting for them to spread.