When F3+B is pressed, all mobs, (with the exception of sheep's body, zombie, skeleton, spider's eyes, enderman's eyes, the mushrooms on a mooshroom, zombie pigman,ghasts, and the blocks an enderman holds), become invisible. Tested in the overworld and the end.

Side Note: "invisible" mobs still block vision of whatever mob is behind them, but not the terrain. So, if I see a zombie through a spider, the part of the zombie that would not be visible if the spider was not invisible can not be seen, almost as though the zombie was partially invisible.

Side Note 2: Invisible mobs will make water behind them invisible. See Bug 5 (pic).

No other entities or blocks appear to be affected