Cron daemon reporting "unknown user 'tomcat6'" every day since I removed tomcat
Solution 1:
This is the logrotate software (that, as the name says, "rotate" logs) complaining about a non-existent user configured to rotate your tomcat logs.
Try searching the /etc/logrotate.d/ directory for something like "tomcat" or "tomcat6" and remove the file if you find it, it's useless now that tomcat is not installed anymore.