A wordpress plugin that allows you to secure your site from changes. You can prevent changes to your wp-admin, wp-content, wp-includes directories. Use this plugin if you got a nasty virus making changes to your site, cos it won't be able to make changes if the site is non writable. There are some cons though, you won't be able to upgrade if the site is completely non writable. When you do want to upgrade, make everything writable beforehand.