A challenging data set for evaluating part-of-speech taggers

This data set contains 2,227 sentences, with a part-of-speech (POS) tag specified for a single word in the sentence. The data file is a tab-separated text file where each row  (after the header row) is formatted as follows:
sentence POS tag (optional) motivation
Note that, in the sentence (= a string of space-separated characters), the POS-tagged word is indicated by the POS tag in brackets, placed just after the word to which it refers. Example:
The road bends [VERB] to the right . VERB
In this example, the optional motivation is not included, as the tagged word can easily be identified as being a verb.