Automatic table row numbers

\documentclass{article}
\usepackage{array,etoolbox}
\preto\tabular{\setcounter{magicrownumbers}{0}}
\newcounter{magicrownumbers}
\newcommand\rownumber{\stepcounter{magicrownumbers}\arabic{magicrownumbers}}
\begin{document}

\begin{tabular}{@{\makebox[3em][r]{\rownumber\space}} | r}
  Something \\
  Other stuff \\
  MAGIC!
\end{tabular}

\end{document}

output

If you want to start with the second row use

\documentclass{article}
\usepackage{array,etoolbox}
\preto\tabular{\setcounter{magicrownumbers}{0}}
\newcounter{magicrownumbers}
\def\rownumber{}
\begin{document}

\begin{tabular}{@{\makebox[3em][r]{\rownumber\space}} | r}
  Something 
  \gdef\rownumber{\stepcounter{magicrownumbers}\arabic{magicrownumbers}} \\
  Other stuff \\
  MAGIC!
\end{tabular}

\end{document}

output

If one wants a heading for the columns use:

\documentclass{article}
\usepackage{array,etoolbox}
\preto\tabular{\setcounter{magicrownumbers}{0}}
\newcounter{magicrownumbers}
\newcommand\rownumber{\stepcounter{magicrownumbers}\arabic{magicrownumbers}}
\begin{document}

\begin{tabular}{@{\makebox[3em][r]{\rownumber\space}} | r}
\multicolumn{1}{@{\makebox[3em][r]{ID~}} | r}{\emph{whatever}}\\    
        Something \\
        Other stuff \\
        MAGIC!
\end{tabular}

\end{document}

You could just make it use a counter...

\documentclass{article}
\newcounter{magicrownumbers}
\newcommand\rownumber{\stepcounter{magicrownumbers}\arabic{magicrownumbers}}
\begin{document}
\begin{tabular}{l|r}
  \rownumber & Something \\
  \rownumber & Other stuff \\
  \rownumber & MAGIC!
\end{tabular}
\end{document}