Bukkit

@Roflzack

The plugin has more features than you think!
You can copy/cut/paste any NBT tags. It is very simple, examples are above
You have access to your clipboard by "c" or "buffer": /nbt buffer
Save clipboard to file: /nbt $filename = buffer
or zipped file: /nbt $$filename = buffer

Apply values from clipboard:
example /nbt me = buffer

You can add only existing tags from buffer (template):
example /nbt me += buffer
If buffer has only Health tag, tour hp will change, but all other tags stay the same.

Look for this plugin: http://dev.bukkit.org/server-mods/powereggs/ It allows you to spawn customised mobs with eggs.

Maybe I'll add the command /nbt undo and /nbt redo in the future. Unfortunately, it can not be applied to items.

@CorruptedHelix

Bad things.
The easiest way - to remove the spawner with any offline map editor.
If you remember the coordinates of the block - you can do this:
/nbt x:y:z:world remove (from console)

You can fix it in game with WorldEdit:
1) Set the view distance: tiny (in game potions)
2) Move away from the block
You can edit your offline position from console:
/nbt @Player Pos[0] = X
/nbt @Player Pos[1] = Y
/nbt @Player Pos[2] = Z
or just login with other name ;)
3) Select the area with a wand, which will be spawner, but do not go near them!
4) //replace 52 air

Next time, remember the location of spawner.

@Roflzack

1)

Server can be crashed if you edit mobspawners. Typo in "EntityID" or "Type" has a terrible effects. This is the most dangerous part. And I have found no other reasons for crashing server.

Client will be crashed:

• when they sees fireworks without colors (Fireworks.Explosions[0].Colors = [] )
• when they hovers over item with unexpected tags
  (display.Name = 0 byte ; ench = "string" etc)
• when someone edit "Primary" and "Secondary" tags of the beacon (Primary = 100500 ; effect with this id should exist)
• when player puckup item with CanPickUpLoot = 1 (fixed in 1.4.6?)

I would say that the plugin is safe for you if you are learned the structure of NBT data or strictly follow the tutorials.

2)

It is possible. You can create custom villager tutorial/villagers and then follow video tutorial tutorial/mobspawners

@ravand

tutorial/potions It works fine!

Do you use spout? Or do you have any other nbt plugins?

Tutorial fixed:
tutorial/mobspawners

@gameacid13

@gameacid13

/nbt $filename paste
or
/nbt $filename = buffer
your buffer will be saved to plugins\PowerNBT\nbt\finename.nbt
or use $$filename (filename.nbtz - zipped nbt file format)

@gameacid13

tutorial/villagers

@gameacid13

I've never edited the recipes, but I am sure that they can be changed using the NBT tags. I'll create a tutorial soon