Show current chapter number on each page margin

Here's a possibility using the background package (change the settings according to your needs). Using \AddLabels the tabs are activated; \RemoveLabels deactivates the tabs:

\documentclass[openany]{scrbook}
\usepackage[contents={},opacity=1,scale=1,color=black]{background}
\usepackage[a6paper]{geometry}% just for the example
\usepackage{tikzpagenodes}
\usepackage{totcount}
\usepackage{lipsum}% just to generate text for the example
\usetikzlibrary{calc}

\newif\ifMaterial

\newlength\LabelSize
\setlength\LabelSize{2.5cm}

\AtBeginDocument{%
\regtotcounter{chapter}
\setlength\LabelSize{\dimexpr\textheight/\totvalue{chapter}\relax}
\ifdim\LabelSize>2.5cm\relax
  \global\setlength\LabelSize{2.5cm}
\fi
}

\newcommand\AddLabels{%
\Materialtrue%
\AddEverypageHook{%
\ifMaterial%
\ifodd\value{page} %
 \backgroundsetup{
  angle=90,
  position={current page.east|-current page text area.north  east},
  vshift=15pt,
  hshift=-\thechapter*\LabelSize,
  contents={%
  \tikz\node[fill=gray!30,anchor=west,text width=\LabelSize,
    align=center,text height=15pt,text depth=10pt,font=\large\sffamily] {\thechapter};
  }%
 }
 \else
 \backgroundsetup{
  angle=90,
  position={current page.west|-current page text area.north west},
  vshift=-15pt,
  hshift=-\thechapter*\LabelSize,
  contents={%
  \tikz\node[fill=gray!30,anchor=west,text width=\LabelSize,
    align=center,text height=15pt,text depth=10pt,font=\large\sffamily] {\thechapter};
  }%
 }
 \fi
 \BgMaterial%
\else\relax\fi}%
}

\newcommand\RemoveLabels{\Materialfalse}

\begin{document}

\chapter{Test Chapter One}
\AddLabels
\lipsum[1-2]
\chapter{Test Chapter Two}
\lipsum[1-2]
\chapter{Test Chapter Three}
\lipsum[1-2]
\chapter{Test Chapter Four}
\lipsum[1-2]
\chapter{Test Chapter Five}
\RemoveLabels
\lipsum[1-2]

\end{document}

In April 2002, the web site of the Koma project, Markus Kohm himself presents both commented code for chapter-thumbs, complemented by a pdf as output.

According to him, it should work with every class with \chapter{}-environment and is based on scrpage2. While xcolor allows coloured backgrounds, graphicx provides rotation of the entries to be printed. In difference to the previously presented solution by Gonzalo Medina, the one by Markus Kohm displays the mark only on the odd side (the right one).