Reinstall all currently installed packages from apt
This script, found on ubuntu forums, might help if you rm -rf some important dir and want to repair everything. Obviously it never happened to me.
for pkg in `dpkg --get-selections | awk '{print $1}' | egrep -v '(dpkg|apt|mysql|mythtv)'` ; do apt-get -y --force-yes install --reinstall $pkg ; done