\documentclass[a4paper]{article}

\usepackage{pgfplots}

\pgfplotsset{compat=1.3}

\begin{document}

\begin{tikzpicture}
	\begin{axis}[colorbar,xlabel=x,ylabel=y,enlargelimits=false,3d box=complete]
	\addplot3[surf]
	  {x^2-y^2};

	\addplot3[
		contour gnuplot={contour dir=y,draw color=red,labels=false}, 
		y filter/.code=\def\pgfmathresult{-5}
	] {x^2-y^2};

	\addplot3[
		contour gnuplot={contour dir=x,draw color=blue,labels=false}, 
		x filter/.code=\def\pgfmathresult{5}
	] {x^2-y^2};

	\addplot3[
		contour gnuplot={contour dir=z,draw color=black,labels=false}, 
		z filter/.code=\def\pgfmathresult{25}
	] {x^2-y^2};

	\end{axis}
\end{tikzpicture}

\end{document}

