/var/log/syslog - general system log /var/log/auth.log - system authentication logs /var/log/mail.log - system mail logs /var/log/messages - general log messages /var/log/dmesg - kernel ring buffer messages, usually since system bootup

For more information, see "Tips on keeping your Ubuntu Linux server secure":

http://www.computerworld.com/action/article.do?
    command=viewArticleBasic&articleId=9002691&pageNumber=3

"This article is excerpted from /The Official Ubuntu Book/ by Benjamin Mako Hill, Jono Bacon, Corey Burger, Jonathan Jesse and Ivan Krstic, copyright Prentice Hall. Reprinted with permission of Prentice Hall, all rights reserved."