With the regular tabular environment, you want to use the p{width} column type, as marcog indicates. But that forces you to give explicit widths.
Another solution is the tabularx environment:
\usepackage{tabularx}
...
\begin{tabularx}{\linewidth}{ r X }
right-aligned foo & long long line of blah blah that will wrap when the table fills the column width\\
\end{tabularx}
All X columns get the same width. You can influence this by setting \hsize in the format declaration:
>{\setlength\hsize{.5\hsize}} X >{\setlength\hsize{1.5\hsize}} X
but then all the factors have to sum up to 1, I suppose (I took this from the LaTeX companion). There is also the package tabulary which will adjust column widths to balance row heights. For the details, you can get the documentation for each package with texdoc tabulary (in TeXlive).