Counting. The math I used most often is making sure I don't make a
fence post error when creating loops. As my statistical mechanics professor said, counting is the hardest thing to do in math.
I suspect you wanted to know which branch of mathematics is most applicible. I'd have to go with discrete math, which includes graph theory, discrete optimization, information theory, game theory, complexity theory, etc. The algorithms course every CS major takes is based on discrete math.
It is used when we selected algorithms (e.g. O(n^2) vs O(nlgn)). It is used when we map data sets to structures. It is used when we determine which approach is easiest. It is used when pooling resources. Of course, in most of these cases, we've comfortable enough with the concepts to be able to do it intuitively. Nevertheless, our intuition is based on underlying mathematical proofs.
--Mark