What's Different/.flow/Yume Nikki

Yume Nikki was a bit of an oddity. It didn't really have any action, or any puzzles, or any plot - all it had was a huge, strange dream world for the player to wander. And that was all it needed: its focus on exploration and its unique atmosphere made it a cult classic. Some compared it a horror game, but it didn't exactly try to scare the player. It just presented them with a variety of strange environments and let them decide for themselves whether to be amused, amazed, or afraid. It did, however, inspire many "fangames" with similar formats, one of which was more clearly a horror game...

Complete with unsettling beeping noises.