The best "tool" available is the No Agenda Podcast.
Hosts Adam and John present national and global issues as portrayed by mass media, and systematically DECONSTRUCT the stories. They not only highlight biases, but explore propagandistic elements and discuss how the media intentionally affects the consumers.
Most importantly, they work to identify the MOTIVATIONS for medias' biases in the first place, and demonstrate for their listeners how to be much more critical, skeptical, and analytical in the way they consume the news.
Listen to a few episodes and you might rethink the need for building a tool at all.