Sentiment analysis, or opinion mining, is the process of extracting sentiment from documents or sentences, where the expressed sentiment is typically categorized as positive, negative, or neutral. Many different techniques have been proposed. In this paper, we report the reimplementation of nine algorithms and their evaluation across four corpora to assess the sentiment at the sentence level. We extracted the named entities from each sentence and we associated them with the sentence sentiment. We built a graphical module based on the Qlikview software suite to visualize the sentiments attached to named entities mentioned in Internet forums and follow opinion changes over time.