Label equation with a symbol

Perhaps something like this.

\documentclass[11pt,a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{mathtools}   % loads »amsmath«

\begin{document}
  \begin{equation}\label{eqn:einstein}
    E=mc^2\tag{*}
  \end{equation}
  \eqref{eqn:einstein}
\end{document}

With amsmath, you have two similar commands:

• \tag{label} where label can be any text or symbol. Note, for most symbol mathmode is required, for example: \tag{$\star$}. Here the label would be typeset within parentheses.

• \tag*{label}, in contrast, does not add parentheses, otherwise it works similar to \tag.

For further information have a look at the amsmath user's guide.

In addition to the answers about how to \tag equations with symbols, there are a number of packages that give you access to a bunch more symbols. Like pifont, ifsym, MarvoSym, bbding See p. 71 et seq. of the Comprehensive LaTeX symbols list for details.

For example:

\documentclass{article}
\usepackage{pifont}
\usepackage{amsmath}
\begin{document}
\[
E=mc^2 \label{eq:einstein} \tag{\ding{37}}
\]
As we can see from \eqref{eq:einstein}\ldots
\end{document}

Produces: