Onextrapixel.com has a great article breaking down WordPress as a web development platform. It includes a list of 12 popular CMS plugins and a design gallery of sites using WordPress, from photography to ecommerce to a creative agency, even an online high-end jewelry store. The article also brings up some important points you might want to consider before using WordPress as a CMS solution, as well as a list of great reasons to use WordPress.
View a sample of the WordPress CMS plugins suggested below.
Flutter is a feature rich WordPress CMS plugin that focuses on easy templating for the developer and simplifies content management for the administrators by creating custom write panels that can be fully customized.
Custom Admin Branding
Custom Admin Branding plugin allows you to not only re-brand the WordPress login screen, but also the admin header and footer with your own custom images. This will actually give your client an idea that the CMS is tailored specially for his business.
Multi Level Navigation
WP e-Commerce shopping cart plugin for WordPress is an elegant, easy to use, fully featured shopping cart application suitable for the selling of products, services and fees online. This will come in handy if you want to sell something on your Blog or site; there is no need for a change in CMS.
Check out the rest at The Autopsy of WordPress As CMS with 25 Great Plugins & Designs.
For more plugins, try 20 Must Have WordPress Plugins For You to Get Started Effectively.