WorldFeatures, A Plugin Inspired by this Reddit post. It automagically builds structures from schematics in a customizable way when the world is created.

Version: 3.03

Plugin name by tmcaffeine

Source (be warned, it still has some code from when I was a noobie)

Description:

Save .schematics to your Created folder in-game or download someone else's .schematics to be generated randomly when new chunks are formed. Each has its own customizable chance to spawn.

A video explaining how to set it up can be found here.

A test of the generation can be found here.

Dependency

You will need WorldEdit.jar in your plugins folder for this to work. You can get it at http://dev.bukkit.org/bukkit-plugins/worldedit/

Permissions:

You will need to use PermissionsBukkit or SuperPermsBridge to use these permissions. If you do not have either, it will default to your OP list.

WorldFeatures.commands

This permission allows the player to use every command. Use this wisely.

Commands:

/wf <name> Creates a schematic named <name> of the selected cuboid. A cuboid can be selected by holding seeds or a customized item.

/wf copy <name> Copies and pastes this schematic from your Created folder into your ToUse folder. Your world will automatically start generating this structure without needing a reset.

Installation:

Insert WorldFeatures.jar into your plugins folder.

Customizing:

After starting up the server for the first time, a WorldFeatures folder will be made. Inside this will be three things:

The "Created" folder - this is where the schematics you create in-game will go to.
The "ToUse" folder - this is where you can place yours or someone else's created schematics. Every .schematic needs a matching .yml. This is explained below.
"Settings.yml" This is explained below.

The Settings.yml has 2 things right now:

wandmaterial - number - what should be held when selecting a cuboid for a schematic?
chunkchance - number between 1 and 100 - chance is explained below.

Individual .ymls for schematics

Each schematic needs its own .yml in the ToUse folder (it should be named <schematicname>.yml). If you are loading a .schematic from elsewhere, copy and paste a .yml and change its name. In each .yml should be these things:

dontpaste - The IDs of blocks that you do not want to paste from the schematic. The only ID that works right now is "0" to not paste air.
anywhereminY/anywheremaxY - the minimum and maximum height the structure can spawn at if "place" is set to "anywhere"
chance - When a chunk is loaded, it checks the chance of the chunk to generate something (by chunkchance in the Settings.yml). If it can, it runs through all the schematics and checks this number for each one. If multiple structures are chosen to generate on one chunk, it will randomly choose one of them to be generated. This means that this "chance" is relative and not an exact amount.
maxspawns - the maximum amount of times this .schematic can spawn
basement - if you have blocks that should be below-ground, this is the number of vertical blocks you want underground, so if I had a building of 30 blocks high, and I only wanted 10 to be above-ground, I would put '20' as the "basement".
biome - can be "none" or any of these values:

DESERT
EXTREME_HILLS
DESERT_HILLS
FOREST_HILLS
JUNGLE_HILLS
TAIGA_HILLS
SMALL_MOUNTAINS
MUSHROOM_ISLAND
MUSHROOM_SHORE
FROZEN_OCEAN
FROZEN_RIVER
HELL
ICE_MOUNTAINS
ICE_PLAINS
FOREST
OCEAN
PLAINS
RAINFOREST
RIVER
SAVANNA
SEASONAL_FOREST
SHRUBLAND
SWAMPLAND
TAIGA
TUNDRA
BEACH
JUNGLE

randomrotate - should this schematic randomly rotate? Default is true.
place can be set to any of these values:

ground - the schematic will stick to the ground and not spawn over water
anywhere - it can spawn anywhere within the .yml range of min and max
air - it will only spawn in the air
underground - it can only spawn underground

Changelog

3.0

Finally updated!!
"wandID" turned into "wandmaterial"
More efficient world creation and .yml saving
.ymls are different for the schematics as well. To see the difference, simply create a schematic with seeds and the /wf command in game