Ryan (core WP dev) Boren's post on taxonomy (category & tag) intersections and unions. WordPress has in-built support for complex include / exclude querying.
A funky site listing chocolate shops in London! A nice example of wordpress-geo-mashups in use.
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.
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.
AskApache Password Protect is a WordPress plugin which uses .htaccess and other methods to block some common WordPress attacks. Some of the options get in the way of functionality, but many do not and will help.
WP Security Scan is a WordPress plugin which scans your WordPress installation for security vulnerabilities and suggests corrective actions.
Article on WordPress security by TechCrunch. Good post encouraging people to keep their version up to date, etc.
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.
Black Hat Boot Camp have a list of (currently 64,724) WordPress blogs which can be searched or downloaded in plain text format, for free.