This sentence is true.
Actually, there is more to that statement than appears at first glance. Usually, the above sentence is assumed to be true, which is a natural approach. If we assume that the sentence is true, then the statement that it is true is in fact true, which means that the sentence is true, as we first assumed.
On the other hand, let us now assume the sentence to be false. If the sentence is false, its statement that it is true is actually false. This means that the sentence is in fact false, which agrees with our original assumption.
So we have reached the extraordinary result that the statement is always consistent, regardless of whether or not it is true! In fact, when isolated, the sentence cannot be determined to be either true or false, much like its counterpart, 'This sentence is false, though this one is always consistent within itself, and could be resolved by some external means.'