Suppose you happen to accidentally do sudo chown -R www-data:www-data * whilst the current directory is /var. Now that’s a fairly daft thing to do! but we all make mistakes. There doesn’t seem to be an easy way to correct this other than restore a backup or look at another machine with a similar setup. Rather than having to restore a backup I looked at a machine with a similar setup, the machine has lighttpd, mysql, and sendmail installed on it so I built the following script based on the permissions with those packages installed on a fresh install of Debian Squeeze

#!/bin/bash

echo "Setting Directory ownership" &&
chown -R root:root /var &&
chown -R man:root /var/cache/man &&
chown -R www-data:www-data /var/cache/lighttpd &&
chown -R libuuid:libuuid /var/lib/libuuid &&
chown -R smmta:smmsp /var/lib/sendmail &&
chown -R mysql:mysql /var/lib/mysql &&
chown -R www-data:www-data /var/log/lighttpd &&
chown -R mysql:adm /var/log/mysql &&
chown -R www-data:www-data /var/run/lighttpd &&
chown -R mysql:root /var/run/mysqld &&
chown -R smmsp:smmsp /var/run/sendmail/msp &&
chown -R smmta:smmsp /var/run/sendmail/mta &&
chown -R smmta:smmsp /var/spool/mqueue &&
chown -R smmsp:smmsp /var/spool/mqueue-client &&
chown -R root:staff /var/local &&
chown -R root:mail /var/mail &&
chown -R root:smmta /var/run/sendmail &&
chown -R root:smmsp /var/run/sendmail/stampdir &&
chown -R root:crontab /var/spool/cron/crontabs &&
chown -R www-data:www-data /var/www &&
echo "Setting File ownership" &&
chown root:smmsp /var/lib/sendmail/dead.letter &&
chown root:adm 
 /var/log/auth.log 
 /var/log/boot 
 /var/log/daemon.log 
 /var/log/debug 
 /var/log/dmesg* 
 /var/log/fsck/checkfs 
 /var/log/fsck/checkroot 
 /var/log/kern.log 
 /var/log/lpr.log 
 /var/log/mail.err 
 /var/log/mail.info 
 /var/log/mail.log 
 /var/log/mail.warn 
 /var/log/messages 
 /var/log/news/news.crit 
 /var/log/news/news.err 
 /var/log/news/news.notice 
 /var/log/syslog 
 /var/log/user.log &&
chown root:utmp 
 /var/log/lastlog 
 /var/log/btmp 
 /var/log/wtmp 
 /var/run/utmp &&
chown root:smmsp 
 /var/run/sendmail/mta/smsocket 
 /var/run/sendmail/stampdir/reload &&
chown -f root:smmsp /var/run/sendmail/mta/sendmail.pid &&
chown mysql:adm 
 /var/log/mysql.err 
 /var/log/musql.log &&
chown -f mysql:adm /var/run/mysqld.pid &&
echo "Done."

In addition you will need to set the ownership permissions of /var/spool/cron/crontabs/<user> and /var/mail/<user> depending on the users on the system.