I wear a full set of molten armor.
I use Muramasa, Blue Moon and a Pheonix Blaster as weapons.
I defeated the Eye of Cthulhu, the Eater of World and Skeletron.
I killed a goblin army without ever being in any real danger.
I explored the Dungeon, the Underground Jungle and the Underworld.
What is there left to do?
- What is there left to kill that might still provide a challenge?
- What places are there left to explore that still contain some secrets?
- What ludicrous buildings or contraptions are still to be to build?
Best Answer
Terraria does not have an end. It's a sandbox. Feel free to do whatever you want! You are using a Muramasa, but it's not the strongest weapon, it's actually one of four items needed to craft the strongest one! (Night's Edge)
Go build something great! There are many forums where you can find people building really amazing things in Terraria, like castles, big forts, houses, etc.
Pixel art! Yes, Terraria is just block, blocks and blocks. Various colours, textures - this can serve as a great set of "painting" tools. And what's the canvas? The world!
PvP. Like it was mentioned in other answers, you can fight other players.
Download custom maps - there are tons of custom maps already, with maze games, adventures, role plays.. Just pick one.
Last but not least - wait for the 1.1 update! It's gonna be huge ;)
The release of 1.1 is planned for the first of December, and there are going to be really interesting things!