calculus - haven't found much use for it myself.
Calculus is one of those fun subjects that you don't appreciate it until you need it, then it is amazing.
Vector calculus is used all the time in both graphics and physics. If you want to deform objects in graphics, to make their normals work out you need to figure out how the deformations change the shape and then change the normals appropriately. Same thing in physics, parametric shapes are defined by math functions, and figuring out angles to bounce and otherwise react with requires calculus to figure out the formulas.
... Either that or you can look up the existing formulas as best you can, and just assume that they did it all right instead of actually know the math yourself.
Splines in general are calculus, but again many programmers don't bother to understand the math, just search out a formula they can run against a matrix multiply to get their results.
Many game programmers get by with iterative methods and accumulating values. It is more precise to compute the endpoints and the rates of change, then directly compute the intermediate rather than accumulating the values (and also accumulating the error). But a little bit of simple calculus as you implement the system lets you implement directly and then solve in the middle.
Any time you are using an iterative solution, like accumulating a tiny bit of motion every update, it is usually better to figure out the proper math for it. Accumulated values also accumulate error, and once you've added a tiny number for a few minutes your accumulated error quickly becomes significant. Better for the programmer to find the rate of change (that's usually calculus!) and then write a function to compute the value directly.