WordPress Theme Authenticity Checker
I found this great plugin that scans your themes and checks for any obfuscated malware or signs of malicious code in your WP theme. I think this should be a MUST for any WP blog that uses a 3rd party theme.
Here's the link: http://wordpress.org/extend/plugins/tac/
Remember, hackers make these themes with malware and give them away for free with their main goal being to eventually use that malware to do some nasty stuff to your site / server. I have seen instances where such malware redirects all your search engine traffic, another that actually uses geotargeting to only take traffic from certain countries, etc.
Maybe we should start a WP plugins post? Where we all list our most valuable or favourite plugins? Anyone keen?