How can I respawn the Ender Dragon?

Minecraft 1.8 and lower

Most servers I've seen just reset the whole dimension by deleting the DIM1 (vanilla) or world_the_end (CraftBukkit) folder.

There is another way to respawn the Ender dragon, that will keep the player builds in The End, but it is a bit more complicated: You need an NBT editor (like NBT Explorer). You need to open DIM1r.0.0.mca and edit in Chunk [0, 0] the TerrainPopulated property to 0:

When the chunk is loaded again, the dragon will be re-created. Note that this will also re-create the obsidian tower(s) in that (0, 0) chunk. Also note that when you kill the new dragon, a new exit portal with a new dragon egg will be created – you may want to switch to creative to remove the unnecessary ones.

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From version 1.9, the Ender dragon can be summoned in-game by placing four Ender Crystals on the edges of the exit portal pool, one on each side. More details on the Minecraft Wiki - Ender Dragon

Use Enderspawn

In vanilla Minecraft, spawning in an Ender Dragon Egg won't work:

You can get a plain white spawn egg that reads Spawn Ender Dragon by typing /give [player] 383 1 63, which is the item ID. However, when you try to spawn it, you cannot and it does not really work as anything.
taken from the wiki: Minecraft Wiki Enderdragon

Instead, you can use EnderSpawn - a server plugin that allows you to easily respawn the Ender Dragon, without having to regenerate the End and lose any player constructions.

Use WorldEdit

It can be done from in game with the WorldEdit plugin (which most servers do), without needing to delete the -dim 1 folder, and therefore preserving any structure you built away from the main platform in the End.

It's rather easy:

1. Go through the end portal.
2. //wand
3. Left click on one corner of the end platform
4. Go to opposite corner (at least 200m away) and make a pile of blocks roughly 20 high
5. Right click top block on your pile
6. //regen

And poof, everything in that entire area is as it original was. Hurry and /spawn before the dragon can generate and it wont even be there until a new player comes through the portal.

Use Commands

The following command will summon an Ender Dragon:

/summon EnderDragon ~ ~ ~