As Harrold Cavendish mentioned it can be done with the ragged2e package. In the following example the whole document is set to raggedright by using the document option and the abstract is kept justified by placed inside a justifying environment:
\documentclass{article}
\usepackage[document]{ragged2e}
\usepackage{lipsum}
\begin{document}
\begin{abstract}
\begin{justify}
\lipsum[1]
\end{justify}
\end{abstract}
\lipsum[2]
\end{document}
The following is said of the document option of ragged2e in the manual:
This options sets the complete
document ragged-right by executing a
\RaggedRight at \begin{document}
and the raggedrightboxes and the
footnotes options.