[orgopete]

The -ve charge of phenoxide is resonance stabillised by the aromatic ring.

Look at the pKa of substituted phenols with electron withdrawing and electron donating substituents.

Despite the risk of being flagged a whole lot more times for my opinion on this, I shall persist.

If asked for a quiz or exam, Archer's answer is the one to give.

If wondering about chemistry, I think we should be more skeptical. I am wary of product stabilization being a good reason to explain acidity if I cannot explain it a priori. We should ask whether the HO of phenol is different than other HO groups. For example, the pKa of H2O2, HOCl, choline, CF3CH2OH, etc are all reduced without resonance. The pKa of cyanic acid is quite low, and while it could be given the resonance argument, it also fits well with an increased electron withdrawing effect of a CN group.

Although the acidity of phenol can be explained by a resonance effect (and is the preferred reason for an exam), can we find additional data to support the resonance structures, for example in the UV spectrum. If we argue the greater contributor has the charge on the oxygen, then we should concede that perhaps the resonance structures may not have that important contribution.

The alternate feature to be examined should be the magnitude of the electron withdrawing effect of an sp2 carbon (to carbon). Herein we have lots of examples. Besides the increased acidity of ethylene and the further increase in acidity of acetylene, I argue this effect can be transmitted through atoms, such as an OH or NH. If a CN group is simply an electron withdrawing element attached to an OH in cyanic acid, then its increased acidity can be similar to HOCl. I argue we must be careful to distinguish between expecting a resonance effect and whether it is really present. The acidity of the NH of pyrrole is also increased. While we may be tempted to draw a resonance structure, let me point out that pyrrole is aromatic by virtue of the non-bonded electron participating in the ring current of the aromatic system. The NH bond electron are perpendicular to the aromatic electrons. 

This effect can be seen in many five-membered aromatic heterocycles. If the neighboring atom is even more electron withdrawing, such as nitrogen, the acidity can be further increased. Compounds like benzotriazole are quite acidic and it seems to me the increase in acidity can occur without invoking an a priori resonance stabilization. If the benzene ring and sp2 nitrogen atoms can be electron withdrawing, they can inductively increase the acidity.

In anticipation of the flags I am about to receive, I am not saying resonance does not play a roll. I am saying I am wary of arguing acidity should be increased due to stabilization of an anion as if the inductive effects of the groups attached to the acid can or should be ignored. I think this was the original poster's comment and one to which I would agree.