Below are links and descriptions for the various tools and free resources available to support creation and editing of objects and biomes:

OTGEdit OTGEdit is a graphical user interface that can be used to create, import, edit and generate world presets. Can also convert schematic files to BO3's.
NotePad++ Useful for editing OTG config files.

Used to convert Minecraft Schematics and modify several BO3 objects at once, according to pre-configured rulesets.

BO3_Layouter BO3_Layouter is a right now rather experimental Application to build semi-Random Customstructures out of individual Parts.

OTG Open Terrain Generator itself. Team OTG
Biome Bundle

A huge pack of over 400 original biomes with over 2,000 structures including custom trees, rocks, caves, dungeons, villages and much more.

Team OTG
Tutorial World This simple world is an example of how to use OTG's settings to create a single climate group with mountains, hills, plains, rivers and lakes. Team OTG
Skylands A Skylands world preset with interconnected floating islands and void below. Watch your step! Team OTG
Flatlands A superflat world preset with many different biomes and super sized villages. Team OTG
Void A void world preset with a single floating island at spawn. Team OTG
Dungeons A dungeon dimension generator intended as a tech demo for the new custom structure (BO3) features of OTG. Forge only atm, we still have to port the new structure features for Spigot which may take a while. Team OTG
Dregora Over 190 biomes and 2200 structures including custom trees, rocks, caves, huge dungeons, villages, cities and more. atleast 4000 hours of dedicated work has been put into this project since 2014. NLBlackEagle
Terra Incognita Features many new biomes, climate zones and trees. Initially designed for Terrain Control but should still function with OTG for the most part. JFromHolland
Dungeoneering Uses OTG world generation to spawn different variations of the vanilla dungeons in an otherwise vanilla world. Intended for use on Ultra Hardcore (UHC) PvP servers. (More Info) SidGarcia
Useful Build Tools
Fast Async WorldEdit & VoxelSniper (FAWE) WorldEdit or VoxelSniper will be automatically installed. For exporting schematics and provides useful build commands and brushes.
WorldEditCUI Forge and Spigot Display WorldEdit Selection.
MCEdit MCEdit is a saved game editor for Minecraft.