The Dungeon Defenders Development Kit has finally been released via Steam as a a free DLC. Here's the blurb from the download page:

• Create and edit maps for Dungeon Defenders: These maps can include new game code, assets, and more which will be automatically downloaded online to new players so they never have to leave the game. Create awesome new mods and become famous as their popularity spreads through the online Dungeon Defenders community!
• Access to all of Dungeon Defenders’ source assets: Build upon our vast library of Dungeon Defenders assets for your mod or total conversion using your art package of choice, so you can quickly create new content, animations, or characters without having to start from scratch.
• Access to Dungeon Defenders’ full source code: Change ANYTHING and create your own completely new game as a Total Conversion! You could make a brand new FPS, a team-versus-team online MOBA, or a third person Role Playing Game. The possibilities are endless and the existing Dungeon Defenders code base and content will give you a head start. This is one of the definitive Unreal Engine 3 reference codebases and one of the largest complete codebase to ever be publicly released!
• Utilize automatic online matchmaking for Mods and Total Conversions: Players automatically download new maps and content as needed when they connect to a previously un-played map or mod. Players can search online games for specific mods and Total Conversions will automatically connect, seamlessly matchmaking among themselves.
• Distribute your mods and total conversions: We give you the rights to distribute your creations anyway you wish, as long as you don't include our EXE's.
• Includes the pre-alpha source for a new, 16-Player PvP Capture the Flag mode, giving you a head-start on creating large-scale, competitive game types!
  

These tools are what's needed to finally get the modding scene in Dungeon Defenders kick started.

Similarly, for any budding modders, JeremyStieglitz has created a basic overview over on the official forums to help get you started.

Good luck, guys!