There are many ways to do this. If you're working with an odd number of rows that you want an entry to span over, using blank cell entries is sufficient. Otherwise, nested tabular (or array and/or the use of \multirows (from the multirow package) make things work:

\documentclass{article}
\usepackage{booktabs,multirow}% http://ctan.org/pkg/{booktabs,multirow}
\begin{document}
\begin{tabular}{*{4}{c}}
\toprule
\textbf{Title1} & \textbf{Title2} & \textbf{Title3} & \textbf{Title4} \\
\midrule
\textbf{Ref1} & Val2a & Val3a $\left\{\begin{tabular}{@{\ }l@{}}
Ref1.i \\ Ref1.ii \\ Ref1.iii
\end{tabular}\right.$ & Val4a \\
\bottomrule
\end{tabular}
\bigskip
\begin{tabular}{*{4}{c}}
\toprule
\textbf{Title1} & \textbf{Title2} & \textbf{Title3} & \textbf{Title4} \\
\midrule
\multirow{3}{*}{\textbf{Ref1}} & \multirow{3}{*}{Val2a} & Ref1.i & \multirow{3}{*}{Val4a} \\
& & Ref1.ii & \\
& & Ref1.iii & \\
\bottomrule
\end{tabular}
\end{document}
tikzmark, this type of thing has been done before :) – cmhughes Feb 06 '13 at 17:00multirowpackage, and its associated package,bigdelim? – Mico Feb 06 '13 at 17:14