This texture pack replaces the default circular, low-res, anti-aliased entity shadow with a square, high-res, blurry one. It blends quite well with the smooth block shadows and in my opinion makes the game look a bit better. If it doesn't show up for you, check your "Entity Shadows" setting.
!! WARNING !!
If you have underpowered hardware, do NOT use this pack! The shadow is 1024x1024 pixels, which is way more than any texture in the game. Please don't come crying in the comments saying that it ruined your performance.
Also, this pack had no effect on my game's FPS with normal amounts of entities, so it most likely won't be an issue for you.