Read this article by Alec on Password Security. A nice discussion on that age old feature and problem.
The following code snippet takes me a few years back, when Alec demonstrated me how ill managed a typical large user/pass data could be.
perl -nle 'setpwent;crypt($_,$c)eq$c&&print"$u=$_"while($u,$c)=getpwent' < /usr/dict/words
Thanks Alec!