Blur the text so it's not readable

Using a variation of the technique Dissecting paragraphs with \lastbox described in Section 5.9.6 of TeX by Topic, you can produce a blur effect; the idea is to use two copies for each line and typeset the line and its copies superimposed:

\documentclass{article}
\usepackage{lipsum}

\newbox\flinebox 
\newbox\slinebox
\newbox\mlinebox
\def\duplines{\setlength\parindent{0pt}
  \setbox\flinebox\lastbox
  \ifvoid\flinebox\relax
  \else
  \setbox\slinebox\hbox{\copy\flinebox}
  \setbox\mlinebox\hbox{\copy\flinebox}
  \unskip\unpenalty
  {\duplines}
  \box\flinebox\vspace*{-2.3ex}
  \box\mlinebox\vspace*{-2.3ex}
  \box\slinebox \fi
}

\newcommand\BlurText[1]{%
  \vbox{#1\par\duplines}}

\begin{document}

\BlurText{\lipsum[1]}

\end{document}

In a comment, doncherry suggested horizontally shifting the layers to decrease legibility; this can be done using this definition of \duplines:

\def\duplines{\setlength\parindent{0pt}
  \setbox\flinebox\lastbox
  \ifvoid\flinebox\relax
  \else
  \setbox\slinebox\hbox{\copy\flinebox}
  \setbox\mlinebox\hbox{\copy\flinebox}
  \unskip\unpenalty
  {\duplines}
  \box\flinebox\vspace*{-2.4ex}
  \makebox[\textwidth]{\hspace*{2pt}\box\mlinebox}\vspace*{-2.4ex}
  \makebox[\textwidth]{\hspace*{4pt}\box\slinebox}\fi
}

and defining \BlurText as before, here's the new result of \BlurText{\lipsum[1]}:

And a little modification using some color:

\usepackage{xcolor}

and in \duplines:

  {\color{black!50}\box\flinebox\vspace*{-2.4ex}
  \makebox[\textwidth]{\hspace*{2pt}\box\mlinebox}\vspace*{-2.4ex}
  \makebox[\textwidth]{\hspace*{4pt}\box\slinebox}}

Here is a TikZ-solution.

\newcommand{\blurred}[3]{
\draw [black!#1] node at (#2,#2) {#3};
\draw [black!#1] node at (0,#2) {#3};
\draw [black!#1] node at (#2,0) {#3};
\draw [black!#1] node at (-#2,#2) {#3};
\draw [black!#1] node at (-#2,0) {#3};
\draw [black!#1] node at (0,-#2) {#3};
\draw [black!#1] node at (#2,-#2) {#3};
\draw [black!#1] node at (-#2,-#2) {#3};
}

\newcommand{\blurry}[1]{
\blurred{5}{0.04}{#1}
\blurred{13}{0.017}{#1}
\blurred{10}{0.012}{#1}
\blurred{15}{0.007}{#1}
\blurred{20}{0.003}{#1}
\blurred{10}{0.0015}{#1}
\draw [black!40] node at (0,0) {#1};
}

For example:

\begin{tikzpicture}
\blurry{This text is a little bit blurry.}
\end{tikzpicture}

In order to strengthen the blur effect, you will have to play with the numbers in the \blurry-command.