SentiWordNet is a lexical resource for opinion mining which is publicly available. SentiWordNet assigns to each synset of WordNet three sentiment scores: positivity, negativity, objectivity.
SentiWordNet is described in details in the papers:
SentiWordNet: A Publicly Available Lexical Resource for Opinion Mining
SentiWordNet 3.0: An Enhanced Lexical Resource for Sentiment Analysis and Opinion Mining
A few other resources on sentiment analysis and opinion mining: