Here are a few links that can help you to secure your WordPress sites.
Think you've been hacked?
http://codex.wordpress.org/FAQ_My_site_was_hacked
Security Plugins & Tools:
https://www.wordfence.com/
https://wordpress.org/plugins/all-in-one-wp-security-and-firewall/
Security Articles & Guides
http://codex.wordpress.org/Hardening_WordPress
http://ottopress.com/2009/hacked-wordpress-backdoors
Scanning Tools:
https://sitecheck.sucuri.net/