You can use p{width} column type for a fixed column, text will wrap automatically. For example:
\begin{tabular}{*{5}{p{3cm}}}
\toprule
& Manual Tests & Randoop 2.5 Minutes With GUI Code & Randoop 2.5 Minutes Without GUI Code & Randoop 5 Minutes Without GUI Code \\ \midrule
\% Covered Instructions & \textbf{82.40\%} & 21.25\% & 21.30\% & \textbf{22.10\%} \\ \bottomrule
\end{tabular}
You can also use \shortstack and use \\ for manual line break. For example:
\begin{tabular}{ccccc}
\toprule
& Manual Tests & \shortstack{Randoop 2.5 Minutes\\ With GUI Code} & \shortstack{Randoop 2.5 Minutes\\ Without GUI Code} & \shortstack{Randoop 5 Minutes\\ Without GUI Code} \\ \midrule
\% Covered Instructions & \textbf{82.40\%} & 21.25\% & 21.30\% & \textbf{22.10\%} \\ \bottomrule
\end{tabular}
\shortstack here can be replaced by a small tabular envronment, for better vertical alignment:
\newcommand{\minitab}[2][c]{\begin{tabular}{#1}#2\end{tabular}}
\begin{tabular}{ccccc}
\toprule
& Manual Tests & \minitab{Randoop 2.5 Minutes\\ With GUI Code} & \minitab{Randoop 2.5 Minutes\\ Without GUI Code} & \minitab{Randoop 5 Minutes\\ Without GUI Code} \\ \midrule
\% Covered Instructions & \textbf{82.40\%} & 21.25\% & 21.30\% & \textbf{22.10\%} \\ \bottomrule
\end{tabular}
Furthermore, you can use makecell package for more flexible commands. \shortstack or \minitab can be replaced by \makecell, \thead etc.
% \usepackage{makecell}
\begin{tabular}{ccccc}
\toprule
& Manual Tests & \thead{Randoop 2.5 Minutes\\ With GUI Code} & \thead{Randoop 2.5 Minutes\\ Without GUI Code} & \thead{Randoop 5 Minutes\\ Without GUI Code} \\ \midrule
\% Covered Instructions & \textbf{82.40\%} & 21.25\% & 21.30\% & \textbf{22.10\%} \\ \bottomrule
\end{tabular}
makecellpackage did the trick. – Tiago Veloso Feb 10 '11 at 18:07