The Terraria endgame, what is left to do?

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!

PvP!

Start a multiplayer server, get one or three friends together, organise yourselves into two teams, and then have a race to do something, or just try and kill each other.

You've conquered the world, now go destroy it. One of the ideas I've been toying with is the complete destruction of a world, without hacked items. Dynamite/dig everything out, fill in water and lava, keep some lava to destroy more. Purify the world as the dryad requests, but on a scale that she just never contemplated. All that should be left is a few demon altars, unless you lucked out and have a world where they are all on sand.

I know this is an old question, but go down to your mine and go to hell. Try to find and kill a voodoo demon, get the guide voodoo doll it drops, and after that go near lava and throw the voodoo doll in the lava. The wall of flesh will spawn, kill it and then you will be in hardmode. A lot of new things will be available. You have just started the game my friend. If you need any help on hardmode look it up on Google.