Adding unnumbered sections to TOC

The easiest way to do this is to use \section but to change secnumdepth so that they don't get numbered.

\documentclass{article}
\setcounter{secnumdepth}{0}
\begin{document}
\tableofcontents
\section{ADSF}
\section{Foo}
\end{document}

Another solution using \addcontentsline{toc}{section}{repeat here section title} relies on nameref. In fact I'm numbering every section (chapters and so on) with a label. So you could use:

\section*{Introduction}
\label{sec:intro}
\addcontentsline{toc}{section}{\nameref{sec:intro}}

It's only a local change and is persistent to names' change of your section.

If you are using KOMA-Script the easiest way is to use these built in commands:

\addpart \addchap \addsec

Which work just like \part* \chapter* \section* while also being added to the toc.