You can also do this without any additional packages, by simply using the tabular* environment, and add \extracolsep{\fill} to fill the column. Here is an example of before and after (I also removed the left column spacing in column 1, and the right column spacing after column 2) where the geometry package was used to include the frame:

Custom Column Type:
If you use this type of column often, you can use the array package and define a custom column type as I have done for the last table. So, with this just using L for your column will achieve what you want. This produces results identical to the second example in the above picture.
Code:
\documentclass{article}
\usepackage[showframe]{geometry}
\usepackage{array}% Only needed if you want to be able to use the L column type
\newcolumntype{L}{@{}l@{\extracolsep{\fill}}}% Needs array package
\newcommand*{\Text}{Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nullam porta tincidunt velit, at lacinia ipsum dictum eget.}
\begin{document}\vspace*{0.1cm}
\noindent
\begin{tabular}{l p{5cm} }\hline
abc & \Text\\hline
\end{tabular}
\bigskip
\noindent
\begin{tabular}{\linewidth}{@{}l@{\extracolsep{\fill}} p{5cm}@{}}\hline
abc & \Text\\hline
\end{tabular}
\bigskip
\noindent
\begin{tabular}{\linewidth}{L p{5cm}@{}}\hline% Using the L column type
abc & \Text\\hline
\end{tabular}
\end{document}
tabularxin atableenvironment and just use it in text, prepend it with\notindentto avoid it sticking out the right margin by\parindent. – Werner Nov 21 '11 at 03:15tableenvironment, then you could use\noindent\begin{tabular}... – cmhughes Apr 01 '12 at 19:32