[Update] The Jets and Patriots will also be wearing helmet decals. Those can be seen below. Also, the Giants helmet is not a decal. Rather, it appears that each player wrote out the initials themselves with a sharpie, adding a personal level to the honor (see second picture below).

[Earlier] The New York Giants have announced that they will honor the students and teachers of Sandy Hook Elementary School as well as the community of Newtown, Connecticut with a decal on the back of their helmets during tomorrow's game. The decal will simply read "S.H.E.S.."

Here are pictures of each of the decals with the Giants on the left, Patriots on upper-right, and Jets on lower-right (click on image for a larger version)...

New York Giants/New York Jets/New England Patriots