Modders, prepare yourselves for this: Hammer is back! Yes, included in the Dota 2 Workshop Tools alpha release, Valve has also included and updated version of Hammer. It's Hammer 2014, if you will.

This alpha is the first release of a new ste of tools that will allow users to create, play, and share custom maps and game modes for Dota 2. This release is "developer focused" and has some lofty system requirements. As it's an alpha, you should expect to see some instability and bugs associated with the release. However, they are looking for feedback from the community in order to improve the tools for a wider release.
  • Completely New Tools - The new suite of tools support creating and importing content for custom games. This set of tools requires a 64-bit version of Windows, Direct3D 11 compatible GPU, and opting into the Steam Client Beta.
  • Steam Workshop Submissions - This early version includes the ability to upload and distribute custom game modes through the Workshop. This new automated process allows players to subscribe to an author's custom game mode to automatically keep current as updates are released by the author.
  • Testing Custom Game Modes - This alpha release is focused on getting the tools to developers first. Currently playing custom games requires a 64-bit version of Windows and a Direct3D 11 compatible GPU. In upcoming future releases we will extend support to more systems (including 32-bit and Direct3D 9) and make it easier for players to download, find matches for, and play user created game modes.

The download is roughly 6GB in size. You have two choices on how to obtain it:
Option 1
  • In your Steam Settings under Account you must opt into Steam Beta Update participation.
  • Right-click on Dota 2 in Steam and select View Downloadable Content.
  • Check the box in the Install column next to Dota 2 Workshop Tools.
  • Click Close. The required content will begin downloading.
  • In Steam, go to the Library menu and click on Tools and install the Dota 2 Workshop Tools.

Option 2
  • In your Steam Settings under Account you must opt into Steam Beta Update participation.
  • Launch Dota 2 in Steam.
  • Click the Play tab and then select Custom Games.
  • Click the Install button.
  • In Steam, go to the Library menu and click on Tools and install the Dota 2 Workshop Tools.

Valve has also set up some detailed documentation for the Dota 2 Workshop Tools on their Valve Developer Community Wiki.