I use the following free WP plugins in my adult wordpress sites.
- Akismet : For moderating blog comments and trackback spam
- Yoast WordPress SEO : This plugin is all you need to cover all the SEO needs of your post
- Backup : Simple and easy backup
- Better WordPress Google XML Sitemaps : Best plugin to generate the sitemap of your website
- Login Lockdown : To protect your website from by restricting the rate at which failed logins can be re-attempted from a given IP range
- No Self Pings : Keeps WordPress from sending pings to your own site
- Redirections : Manage all your 301 redirects and monitor 404 errors
- SEO Friendly Images : Automatically adds alt and title attributes to all your images
- WordPress Editorial Calendar : To add your weekly post all at one time while scheduling some of them such that your website gets updated regularly and keeps Google happy
- WP Smush : Reduce the thumbnail sizes of all your images which improves site speed and therefore improving site performance