Sentiment Analysis (SA), also called Opinion Mining, is currently one of the most studied research fields. It aims to analyze people’s sentiments, opinions, attitudes, emotions, etc., towards elements such as topics, products, individuals, organizations, and services. Different techniques and software tools are being developed to carry out Sentiment Analysis. The goal of this work is to review and compare some free access web services, analyzing their capabilities to classify and score different pieces of text with respect to the sentiments contained therein. For that purpose, three well-known collections have been used to perform several experiments whose results are shown and commented upon, leading to some interesting conclusions about the capabilities of each analyzed tool.