It really depends on your domain. Knowing more math is never a bad thing. It provides you tools for you to model and solve problems. That said, two areas that I

It really depends on your domain. Knowing more math is never a bad thing. It provides you tools for you to model and solve problems. That said, two areas that I felt most broadened my horizons are probability and linear algebra. They are pretty ubiquitous topics that come up regularly in more sophisticated and exotic algorithms.