given prime factors of a number, what is the fastest way to calculate it's proper divisors?
Mathematica
Union[Prepend[Times @@@ Rest@Subsets@l, 1]]

Usage
I'm assuming that repeated factors would be repeated in the list of factors, as below:
l = {2, 2, 3, 5, 5, 11, 11, 11, 31, 37, 617};
Union[Prepend[Times @@@ Rest@Subsets@l, 1]]//AbsoluteTiming


This routine is much slower than the built in function Divisors, which has to first factor the Integer.  I am fairly certain that Divisors is compiled.
Divisors[282584210700]; // AbsoluteTiming


{0.000262, Null}