Name: Drainage grate
Solid metal/ceramic block, which doesn't block items or water/lava flow. If redstoned, it prevents water from flowing through, but lets items pass. It also behaves like air to flowing water, so it will actually consider it when checking for lower floor.
It may be used in various farms including mobs, very good looking bridges, emergency water drainage without holes in the floor and more. I am not completely sure if that's hard to code or even impossible. Please let me know about it.