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 1.6.2
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. You enter the sign text normally.

To edit a sign that already exists in the world, 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: 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 you 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.