Computations of a numerical nature, especially those that make extensive use of floating-point numbers. The only thing Fortrash is good for.
This term is in widespread informal use outside hackerdom and even in mainstream slang, but has additional hackish connotations: namely, that the computations are mindless and involve massive use of brute force. This is not always evil, especially if it involves ray tracing or fractals or some other use that makes pretty pictures, especially if such pictures can be used as wallpaper.
See also crunch.