Using PMX outside MMD

First, there are native MMD-like softwares (for animation/rendering, no modeling capabilities):

• MikuMikuMoving
• NexGiMa
• MikuMikuDayo
• nanoem

And there are MMD-compatible softwares based on web technologies (babylon-mmd, three.js, etc.):

• MMD modoki
• Reze Studio
• AnimaStage

Alternatively, you can import PMX via addons in non-MMD softwares. Some models prohibit this use case, so you should be careful not violate terms of use. There are many softwares that can import MMD models.

• Blender with MMD Tools
• Unity with MMD4Mecanim
• UE5 with IM-for-UE5
• Godot with godot-pmx or Godot-MMD
• Maya with mmdpaimaya or Maya MMD Tools
• Cinema 4D with C4D MMD Tool