Luo, Osborne and Wang make the following data set available:
They crawled 30 million English-language tweets and then had 7 people use a search engine to call up results. The results showed 100 tweets and the people had to classify each of the 100 for whether it was (a) opinionated about the query, or (b) not opinionated.
There were 50 queries resulting in 5,000 annotated tweets.
Read their paper here: