problem: What is the product of all constants $k$ such that the quadratic $x^2 + kx +15$ can be factored in the form $(x+a)(x+b)$, where $a$ and $b$ are integers?
solution: If $x^2 + kx + 15= (x+a)(x+b)$, then \[x^2 + kx + 15 = x^2 + ax +bx +ab = x^2 +(a+b)x + ab.\]Therefore, we must have $ab = 15$, and for any such $a$ and $b$, we have $k = a+b$.   There are four pairs of integers that multiply to 15.  They are 1 and 15 (which give $k=16$), 3 and 5 (which give $k=8$), $-1$ and $-15$ (which gives $k=-16$), and -3 and -5 (which give $k=-8$).  The product of these four possible values of $k$ is \begin{align*}
  (16)(8)(-16)(-8)& = (2^4)(2^3)(-2^4)(-2^3)\\
  & = 2^{4+3+4+3} \\&= 2^{14}\\& = 2^{10}\cdot 2^4 = (1024)(16) = \boxed{16384}.
  \end{align*}