SentiWordNet is a lexical resource for opinion mining. 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

How to obtain SentiWordNet

The current "official" version of SentiWordNet is 3.0, which is based on WordNet 3.0.

SentiWordNet is distributed under the Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0) license.
Among the other possibilities, this license allows the use of SentiWordNet in commercial applications, provided that the application mentions the use of SentiWordNet and SentiWordNet is attributed to its authors.

Click here to download SentiWordnet 3.0

micro-WordNet-Opinion 3.0

micro-WordNet-Opinion 3.0 is the automatic mapping of the micro-WordNet-Opinion corpus to WordNet 3.0.

SentiWordNet has been used in...

Check Google for a list of the papers that use SentiWordNet 3.0

Check Google for a list of the papers that use SentiWordNet 1.0

Sample code

Here you can find a demo java class, kindly provided by Petter Tönberg, that uses SentiWordNet to approximate the sentiment value of a word with a label.

P.S. Note that in this piece of code there is no word sense disambiguation and the scores are approximated with labels, so it's very basic.

User feedback

The SentiWordNet interface provided the possibily to accept user feedback on the values assigned to synsets.

Feedback collected from June 4, 2010 to May 13, 2013. - Release report: "The User Feedback on SentiWordNet".

The feedback collected on SentiWordNet is distributed under a Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0) license.