Prime on the product symbol

The amsmath package has a \sideset command for this purpose:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{equation*}
  \sideset{}{'}\prod_{n=-k}^k \frac{a_n}{n}
\end{equation*}

\end{document}

The general syntax is

\sideset{_a^b}{_c^d}\sum

producing

and works on any large operator \prod, \sum, etc.

You have to detach the superscript from the operator. Below I've inserted a \vphantom that is the size of \prod:

\documentclass{article}

\begin{document}

\[
  \prod_{i=0}^n f_i \quad \prod{\vphantom\prod}'
\]

\end{document}