Not serious:
Modern Warfare 3.
Although more of a teaching aid than an accessory, it shows real-life weapons and scenarios, while providing tons of trigger time, all from the comfort of a 12yo's parents' basement.
Semi-serious
The beautiful Swiss Sig57 surplus bayonnet
Adds functionality of a pokey thing to a shooting thing.
Serious:
Bought my first red dot sight this year. Can see now why a red dot, be it Vortex, Aimpoint, etc., are so popular. While I still like irons, having a good red dot on a rifle is a pleasure.