Anti-malware

Good all-rounders: anti-crackers, bot-busters, scanners, monitors, and alerters:

AntiVirus	3.1.1
Block and alert on a wide range of attempted exploits and spam injections.
http://wordpress.org/extend/plugins/antivirus
AskApache Password Protect	3.1-alpha
Curb attacks with ModSecurity, mod_rewrite, mod_alias, and other security features.
http://wordpress.org/extend/plugins/askapache-password-protect
BulletProof Security	3.2-bleeding
Secures key files and protects against attacks including XSS and SQL injections.
http://wordpress.org/extend/plugins/bulletproof-security
Exploit Scanner	3.1.1
Searches files, plugins, and database for suspect code, reporting any changes.
http://wordpress.org/extend/plugins/exploit-scanner
Secure WordPress	3.1.1
Addresses WordPress-specific information leaks and blocks bad queries.
http://wordpress.org/extend/plugins/secure-wordpress
Ultimate Security Checker	3.1.1
Scans for known threats, repairing problems manually or automatically.
http://wordpress.org/extend/plugins/ultimate-security-checker
WordPress File Monitor	3.0.5
Monitors the web files and alerts about anything added, deleted, or changed.
http://wordpress.org/extend/plugins/wordpress-file-monitor
WordPress Firewall 2	3.0.5
Investigates web requests with heuristics to identify and stop obvious attacks.
http://wordpress.org/extend/plugins/wordpress-firewall-2
WP Security Scan	3.1.1
Scans for vulnerabilities, suggesting corrective actions and includes a prefix changer.
http://wordpress.org/extend/plugins/wp-security-scan