Vowpal Wabbit is a general-purpose machine-learning library. Even though it was originally created at Yahoo! Research over a decade ago, we still want to mention it to highlight that it continues to be the place where many of the newest machine-learning techniques get added first. If you're interested in machine learning, you may want to keep an eye on the innovations in Vowpal Wabbit. Note also that Microsoft has shown a deeper interest in Vowpal Wabbit in recent years, employing a main contributor and integrating it into their Azure offerings, for example in their machine-learning designer and in Personalizer.