Along with the upcoming release of the massive Terraria: Journey's End free expansion, Re-Logic have announced the popular open source modding tool tModLoader will be available on Steam.
Sadly, official mode support is not going to happen with Terraria but tModLoader is well known, free and open source under the MIT license and has been going for some time now. This truly official endorsement is great to see and it will now be available as a free DLC on Steam along with the Terraria: Journey's End free expansion on May 16 (or shortly after).
When it launches, it will be based on the existing 1.3.5 version of the game - as will all of the currently-available mods out there. This way there will be no interruptions when the new Terraria version launches. However, an update for tModLoader to support Terraria 1.4 Journey's End will be coming too.
Since Journey's End will be the final major content update, Re-Logic felt this was a good time to "hand the keys over to the fans". Eventually it will even get Steam Workshop support for use with tModLoader too.