As of last night, the Dungeon editor has been fully released for everyone who has a copy of Legend of Grimrock! Just update the game and the Editor will be yours!
-- Original Story --
The indie developer behind Legend of Grimrock, Almost Human, has announced that the game will be getting "a major update that will enable the gaming community to create their own adventures using a powerful yet easy-to-use toolset." As of today, the free upgrade enters public beta testing!
The Dungeon Editor will allow for the building of new dungeons, and new experiences for Legend of Grimrock. It sports a slick graphical interface with an instant preview while providing very comprehensive modding abilities. Steam Workshop support has been built in for those who have it purchased through Steam to allow easy sharing of user-mods.
A Steam account and a Steam installed version of the game is required for the beta though. If you purchased the game outside of Steam, you will need to wait for the full release to check it out.
"To join the beta right-click on Legend of Grimrock in your Library of games in Steam and select Properties. In the Betas tab opt into the beta using the dropdown there. The Steam Client should then automatically download the beta."
To mark the pre-release, the developers have also provided a trailer for the new add-on, so you can see what it can do for yourself.