Castlevania: Simon's Destiny - freeware, GZDoom.Blasphemer - free software, vanilla-compatible IWAD replacement.The Adventures of Square - freeware, GZDoom.
I'm looking specifically for standalone titles here, not TCs that require some other IWAD to run.
I'm trying to make an exhaustive list of all the indie games that were created using the Doom engine as a base.