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 DIM1\r.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.

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 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 ~ ~ ~

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.