The Mpemba effect, named after Erasto Mpemba, is the observation that, in some circumstances, warmer water can freeze faster than colder water. Although there is evidence of the effect, there is no agreement on exactly what the effect is and under what circumstances it occurs. There have been reports of similar phenomena since ancient times, although with insufficient detail for the claims to be replicated. A number of possible explanations for the effect have been proposed. Further investigations will need to decide on a precise definition of "freezing" and control a vast number of starting parameters in order to confirm or explain the effect (wikipedia).
Did you read the paper from Xi Zhang et al? He proposes a solution in "O:H-O Bond Anomalous Relaxation Resolving Mpemba Paradox". The abstract is below:
http://arxiv.org/abs/1310.6514
Well it is not freezing faster, but it CAN freeze faster.
Usually it is called the Mpemba effect (although there have been reports of similar phenomena since ancient times).
I think there is no final agreement on the how the process exactly takes place, but the an easy and plausible explanation is that the lower temperature water will tend to freeze faster as a "block", while the warmer water will tend to freeze a "single layer" at the top and this further isolates the water mass.
So in a sense, the lower temperature water will ALWAYS freeze faster (until becoming a single ice block), but the warmer water may create faster an ice cube with liquid water inside.
Here you can find a nice paper with some deeper insights:
http://www.rsc.org/images/nikola-bregovic-entry_tcm18-225169.pdf
But this effect is not alone, since the "opposite" also holds, namely, lower temperature boilers can sometimes vaporize water faster than higher temperature boilers (Leidenfrost effect)
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