This tool works by examining individual words and short sequences of words (n-grams) and comparing them with a probability model. The probability model is built on a prelabeled test set of IMDb movie reviews. It can also detect negations in phrases, i.e, the phrase "not bad" will be classified as positive despite having two individual words with a negative sentiment.
I've also provided an API for this tool, details on how to use it are on this page
Author: Vivek Narayanan < email@example.com >