We all spend lot of time writing and promoting our blogs. We should do our bit to keep our blogs secure:

This is one precious plugin: WP Security Scan

Block WP- folders from being indexed by search engines, the best way to block them in your robots.txt file. Add the following line to your list:
Disallow: /wp-*
Directories should not be left open for public browsing- There is a potential problem letting people know what plugins you have, or what versions they are. If there is some known exploit that is linked to a plugin, it could be easy enough for someone to use it to their advantage. Make an empty wp-content/plugins/index.html file or just add this line in your .htaccess file in your root:

Options All -Indexes