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
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.
The SentiWordNet interface provided the possibily to accept user feedback on the values assigned to synsets.
The feedback collected on SentiWordNet is distributed under a Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0) license.