Must-have plugins for WordPress
DDoS protection plugin by 2by2host (requests a copy from 2by2host)
Malware cleaner by Galleonhosting.com (request a copy from 2by2host)
Mandrill (allows to send all emails from the site through the mandrill service)
Simple Trackback Disabler (removes and disables comments and trackbacks)
Facebook Comments (replaces built-in comments with FB comments. A great way to get rid of spam and improve performance and user conversions)
and any other plugins that are not being used!
Remove any unused themes as well!