crosshair in the middle of a page

You can use eso-pic to place content in the ForeGround in the centre of every page:

\documentclass{article}

\usepackage{eso-pic,xcolor,lipsum}

\AddToShipoutPictureFG{%
  \AtPageCenter{%
    \color{red}%
    \makebox[0pt]{\rule{250pt}{2pt}}%
    \makebox[0pt]{\rule[-250pt]{2pt}{500pt}}%
  }%
}

\begin{document}

\lipsum[1-50]

\end{document}

This uses TikZ and places a cross hair at the page center, extending some size to the top/bottom and left/right margins, the dimensions of this is used from the 1st and 2nd arguments of the macro \crosshair. The 3rd argument is meant for colour etc. settings.

Please compile twice to get the node anchor placement correctly.

\documentclass{article}

\usepackage{tikz}

\usetikzlibrary{calc}

\usepackage{blindtext}

\newcommand{\crosshair}[3]{%
\begin{tikzpicture}[remember picture,overlay,opacity=0.2]
  \draw[black, line width=2pt,#3] ($(current page.center)-(#1,0)$) -- ($(current page.center)+(#1,0)$);
  \draw[black, line width=2pt,#3] ($(current page.center)-(0,#2)$) -- ($(current page.center)+(0,#2)$);
\end{tikzpicture}%
}

\begin{document}
\crosshair{5cm}{10cm}{blue}
\blindtext[10]
\crosshair{10cm}{2cm}{red,dashed}
\end{document}

Update for crosshairs on every page with everypage package

Use the \AddEverypageHook macro in order to add some code that should be used on every page, i.e. place the \crosshair macro inside \AddEverypageHook as has been done in the code below.

\documentclass{article}

\usepackage{everypage}
\usepackage{tikz}

\usetikzlibrary{calc}

\usepackage{blindtext}

\newcommand{\crosshair}[4][opacity=0.2]{%
\begin{tikzpicture}[remember picture,overlay,#1]
  \draw[black, line width=2pt,#4] ($(current page.center)-(#2,0)$) -- ($(current page.center)+(#2,0)$);
  \draw[black, line width=2pt,#4] ($(current page.center)-(0,#3)$) -- ($(current page.center)+(0,#3)$);
\end{tikzpicture}%
}

\AddEverypageHook{\crosshair[opacity=0.5]{5cm}{10cm}{blue}}

\begin{document}
\blindtext[10]
\end{document}

You can use also \watermark or \leftwatermark and \rightwatermark for even/odd pages, or \thiswatermark for a particular page:

\documentclass{article}
\usepackage{watermark,xcolor}
\usepackage{lipsum} % dummy text
\begin{document}
\thiswatermark{\color{red!50}\rule{.5\textwidth}{0pt}%
\rule[\dimexpr-1\textheight-\headsep]{1pt}{1\textheight}}
\lipsum[1-6]
\end{document}