WordPress is one of the best CMS as it can be tailored to suit any needs and the best part is that it is good enough for SEO and hosts a large database of plugins that enhance its functioning. Here I have listed some plugins that can make WordPress to act more as a CMS rather than being blog-centric.
Its a powerful all-in-one plugin to manage a wide range of SEO tasks, option to add Google section targeting, add tracking codes for Google analytics and like services, welcome first time visitors and a hell more. If you haven't check out this plugin, then you are missing something you shouldn't. If you still think you don't need it, then visit the plugin's homepage.
Ryans Simple CMS
This plugin re-designs the dashboard using CSS so that your client only gets to see what he needs to see and all others things are hidden so that he might not mess up with something or the other. To download or login into a demo blog, visit Pixopoint Simple CMS
WordPress Database Backup
With this you can schedule backup of WordPress database and backups will automatically be mailed to the email specified. This plugin only features backup of your database and not your files you may have uploaded. Plugins that provide database backup along with files can be find here and a beta version of another plugin here. I myself use the WordPress Database Backup plugin because the backups are mailed everyday and I prefer to take backup of other files myself. No hassles. If you still want a mix, then you can go for Skippy's plugin.
WordPress Automatic Upgrade Plugin
Whenever a new version of WordPress is launched, this awesome plugin by Keith does the job of upgrading the WordPress installation in a matter of clicks. Although I check for updates on his plugin as soon as WordPress changes the way it work and then use this plugin to update my installation. You can download it from here.
You don't need to visit the plugin page anymore to check if you need to upgrade any plugin. Plugin Central will notify you on the dashboard itself. You can have a look at the screenshot to get a idea. If you don't want to update a plugin, there is a option of adding that plugin to Ignore list too. There is no need of updating the plugins one by one, you can update them all with a single click and the coolest part is it can even install the plugin by just its name (if found on the WordPress repository, of course). You can save time while setting up CMS for a client as you only need to give this plugin the list of plugins to install and expect the rest to be done by it. Get it from Prelovac.
Provide brute force protection to your client's site. This plugin does the job of locking the login after a number of unsuccessful attempts to login from a specific IP address. Good plugin for sure. You can download it for WordPress 2.5+ and older versions from Bad Neighborhood.
Branded Login Screen
A custom login screen will look cool and will give the client a feeling that they really have what they wished for. You still need to design a image on your own as this plugin only does the job of displaying them and not creating it. More instructions can be found on Kerry's site.
If you know of any WordPress plugin that can enhance its capability to act as the ultimate CMS, then share with us.