You can insert content into the ToC using \addcontentsline{<toc>}{<section>}{<title>} and \addtocontents{<toc>}{<stuff>}:

\documentclass{article}
\newcommand{\inserttocgap}{\addtocontents{toc}{\protect\addvspace{2\baselineskip}}}
\begin{document}
\tableofcontents
\section*{Introduction}
\addcontentsline{toc}{section}{Introduction}\inserttocgap
\section{A section}
\section{Another section}
\inserttocgap
\section*{An unnumbered section}
\addcontentsline{toc}{section}{An unnumbered section}
\end{document}
The above works for article, but can be updated to work for book or report that defines \chapter. Just use
\addcontentsline{toc}{chapter}{<title>}
\inserttocgap inserts a 2\baselineskip gap. In order to survive writing to the ToC (and expansion), \protect it.