Diffusion filters, I absolutely love them and use all different kinds. I'll be going over the White Mist & Black Mist from Hoya. These live on my film cameras but you could also use them in your digital SLR cameras, mirrorless or any cameras where you want some diffusion to add a little bit of a character to your images.
One of the reasons I love shooting 35mm film is to take advantage of the tiny negative. 24mm x 36mm is tiny compared to 6cm x 6cm and specially next to 8x10. Below we can see a side by side comparison of sizes of the negatives (left - 8x10 negative, right - strip of 135 film):