G.Skill is renown for their gaming memory range, as well as being at the forefront of overclocking records. Memory from them currently holds the record for the fastest frequency, at DDR3 4404MHz which you can see here, and whilst that is certainly impressive in its own right, most memory records are built on a single stick in the machine, and this helps to reduce the stress on the overclock and is not as close to a real world comparison as say running multiple sticks. (disregarding the fact that often advanced cooling such as LN2 is used).
Today however they have achieved an even more impressive feat, using a G.Skill TridentX Quad Channel Memory Kit, one overclocker has been able to get the sticks to the record breaking frequency of DDR3-4072MHz, LN2 was again used to reach this massive frequency. The processor used in the Asus Rampage Board also had features disabled to make the overclock stick, so hitting frequencies such as this is still a long way off from us mere mortals. However knowing the competitiveness of overclocking, I don't think it will be too long until the record is broken.
The Validation of the Overclock is below.