http://www.cs.tut.fi/%7Erammer/aide/manual.html
The beginnings of the manual for AIDE (Advanced Intrusion Detection Environment), the free replacement for Tripwire.
http://ocaoimh.ie/2008/06/08/did-your-wordpress-site-get-hacked/
Donncha writes a useful article on detecting if your site has been hacked, monitoring systems to check for hacks, and how to secure your site to avoid future hacks.
http://www.la-samhna.de/samhain/
Samhain is a multiplatform, open source host-based intrusion detection system (HIDS). Samhain provides file integrity checking, as well as rootkit detection, port monitoring, detection of rogue SUID executables, and hidden processes.
http://sourceforge.net/projects/tripwire/
Tripwire monitors files on the filesystem for changes as a method of intrusion detection. Similar systems include AIDE and Samhain. Useful for detecting web site hacks.
http://wordpress.org/extend/plugins/wp-security-scan/
WP Security Scan is a WordPress plugin which scans your WordPress installation for security vulnerabilities and suggests corrective actions.
http://www.techcrunch.com/2008/06/11/my-blog-was-hacked-is-yours-next-huge-wordpress-security-issues/
Article on WordPress security by TechCrunch. Good post encouraging people to keep their version up to date, etc.
http://www.noupe.com/how-tos/wordpress-security-tips-and-hacks.html
An article from Noupe.com on securing WordPress installations. Some general tips such as restrict by IPs, block repeated failed logins, disallow directory listings, block access to wp-config.php, etc.