The 2003 Troika RPG Temple of Elemental Evil was not great. It was a faithful reproduction of the classic D&D module—a little too faithful, in my opinion—but it was also riddled with bugs. A persistent community of modders spent the following decade smoothing out the rough spots and restoring cut content, but there were limits to what could be done with it because nobody had access to the world editor. But according to an anonymous leak to the RPG Codex, it was there all along.
The process for accessing the editor is actually quite simple. Begin by downloading the official Polish 2.0 patch, and yes, it must be the Polish patch; grab it here. Then, create an empty directory, and an empty text file called ChangeLog.txt. Finally, run the patch and select the newly-created directory as the installation destination. That's it: The world editor—apparently called WorldEd—is now installed.
The discovery is probably more noteworthy as an anthropological curiosity than for any practical impact it might have, although the general consensus is that despite being an imperfect piece of software, it will make life simpler for modders. Also very interesting is that shortly after the leak was made public, the Circle of 8, the modding group that used its own toolset to create and release updates as recently as last August, claimed in its own forums that it been aware of WorldEd's presence in the game "for some time," but kept that knowledge to itself for fear of possible legal trouble.
Historical oddity and forum drama aside, there's some legitimate excitement about what this could mean beyond simply improving the original game. I have no idea how likely it is to happen, but it does appear to open up the possibility of new content being created in the ToEE engine: The Circle of 8 described it as "quirky and crashy", but also acknowledged that "it does indeed open up addition modding possibilities," while the Codex itself has opened up a new Temple of Elemental Evil modding subforum.