FlatSigns

Overview
FlatSigns is a mod, by Myrathi, providing new sign functionality that allows signs to be placed flat on the ceiling or the floor (via sneak click).

You can also make sponges to edit existing signs.

Minecraft 1.5.1
In the early versions of the mod, signs can be placed on ceilings and, if sneak is held, flat on the floor.

Minecraft 1.5.2
As of v1.3.1 build 13, sponges and sponge wipes can be created and used to edit signs in-place.

How It Works
There are no "new" sign blocks - you simply "place" a normal sign on the ceiling or sneak-place it on the floor. Everything should work as you'd expect!

How do you edit a sign? You right-click a sponge or sponge wipe on it.

To make a sponge (which is usually only available in creative mode), simply throw yellow wool into water or onto the ground when it's raining. It'll soak up the water. SHLOOP! You have a sponge!

The problem with sponge is that it's not very durable. The solution? You make it into sponge wipes! Simply craft a sponge with a piece of cactus green (to give it a rougher side) et voilà: you have something akin to a steel-wool pad!

You can also place sponge beside a sign (without editing it) by sneaking while placing the sponge. It won't be "used" but will, instead, appear as a block.