Adding space between columns in a table

The parameter to act on is \tabcolsep, whose value is usually 6pt. So

\setlength{\tabcolsep}{12pt}

will double the intercolumn space. The parameter stores half the space between columns: in LaTeX each column is preceded and followed by a horizontal space \tabcolsep wide.

You can add space by inserting @{\hskip whatever} between the column specifiers, as in

\begin{tabular}{l@{\hskip 1in}c@{\hskip 0.5in}c}
  One&Two& Three\\
  Four& Five& Six
\end{tabular}

Actually, you can add whatever you like between the columns in place of the usual intercolumn space, e.g.,

\begin{tabular}{l@{ or }c@{\hskip 0.5in}c}
  One&Two& Three\\
  Four& Five& Six
\end{tabular}

If you need to adjust the spacing for just one table you could also add/remove some space before the table and restore previous value after it:

\addtolength{\tabcolsep}{-1pt}    
\begin{tabular}{cc}    
text & text    
\end{tabular}    
\addtolength{\tabcolsep}{1pt}