Nothing that is known to exist
Relativity does not allow for a massive particle to travel at the speed of light, but it doesn't prevent a particle from travelling faster than light. Such a particle has been called a Tachyon. No such particle has ever been observed. There are good reasons for believing that they don't exist.
Such a particle would be extremely strange. The mass of such a particle would not be positive, it would be imaginary! However you could not use such a particle to send a message to the past. The particles can't be localised, which means that you can't detect them as being "at" a particular place at a particular time.
In conclusion, such particles have never been detected, probably don't exist, and couldn't be used to send messages.
read more at http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/tachyons.html