\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning}

\begin{document}
\begin{tikzpicture}[minimum width=2cm,minimum height=1cm,node distance=1cm]
	\node[draw,rectangle] (cooja) {COOJA};
	\node[draw,rectangle,below left=of cooja] (mspsim) {MSPSIM};
	\node[draw,rectangle,below right=of cooja] (avrora) {avrora};
\end{tikzpicture}
\end{document}
