FlatSigns

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).

Sponges are also added to edit existing signs.

Minecraft 1.7.10
As of v2.0.0 build 17, the mod supports the 1.7.10 client.

Minecraft 1.6.2
As of v1.4.0 build 15, the mod supports the 1.6.2 client.

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.

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.

How It Works
There are no "new" sign blocks: placing a normal sign item on the ceiling or sneak-placing one on the floor will result in the new, flat style of sign. Sign text can be entered normally.

To edit a sign that already exists in the world, the player can 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: the wool will soak up the water, resulting in a Sponge Block.

Given that Sponge is not very durable, there is the option to make it into Sponge Wipes: simply craft a Sponge with a piece of Cactus Green (to give it a rougher side), giving 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.