Hyperref in toc without page number

hyperref redefined \contentsline to take four arguments instead of the usual three. Additionally, since you're working with ToC-related stuff (which are written to file), you need to be careful about expansion. Use it in the following way:

\documentclass{report}
\usepackage{hyperref}
\begin{document}
\tableofcontents
\chapter*{Introduction}
\addtocontents{toc}{\protect\contentsline{chapter}{\protect\numberline{}Introduction}{}{}}
\end{document}

The fourth argument supplies the hyperref link (or anchor). Since it is empty, the ToC-entry won't be hyperlinked.

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Adding a hyperlink depends on the type of link. In your case, you can use the following:

\documentclass{report}
\usepackage{hyperref}
\begin{document}
\tableofcontents
\clearpage\phantomsection
\chapter*{Introduction}
\addtocontents{toc}{\protect\contentsline{chapter}{\protect\numberline{}Introduction}{}{chapter*.\thepage}}
\end{document}

Each \chapter* can be accessed using chapter*.<current page>, which is passed to the last option of \contentsline. Since the \phantomsection/mark is made on the current page, chapter*.\thepage results in an appropriate expansion of the current \chapter*.