How to a put a repeated text in the left margin of a page, like a header or footer, but on the side

The background package could be an option:

\documentclass{letter} 
\usepackage{background}
\usepackage{lipsum}% just to generate text for the example

\SetBgAngle{0}
\SetBgScale{1}
\SetBgColor{black}
\SetBgOpacity{1}
\SetBgPosition{.01\paperwidth,-.5\textheight}
\SetBgContents{\fbox{\parbox{3cm}{%
Return Address:\\
John Q. Citizen\\
500 Park Place\\
Boston, MA. 01234}}%
}

\begin{document} 

\lipsum[1-4]

\end{document}

There are a couple of packages that provide absolute positioning of items on a page. For example, the eso-pic package allows for foreground or background placement of arbitrary stuff at page shipout. The following minimal example places a similar \fbox{\parbox{..}{...}} to what is included in @Gonzalo's answer on the foreground of all pages in your document:

\documentclass{article}
\usepackage{eso-pic}% http://ctan.org/pkg/eso-pic
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\begin{document}
\AddToShipoutPictureFG{%
  \AtTextCenter{%
    \hspace*{-0.45\paperwidth}\fbox{\parbox{3cm}{%
      Return Address:\\
      John Q. Citizen\\
      500 Park Place\\
      Boston, MA. 01234}}%
  }
}
\lipsum[1-6]
\end{document}

Using the starred version \...FG*{<stuff>} only includes <stuff> on the foreground of the current page.

Also see the documentations of the textpos package and that of tikz.