The power of WordPress is in the community and the ability to extend WordPress through the use of themes and plugins. (*note that plugins can only be used in WordPress.org sites, not WordPress.com) In no means is this meant to be a complete listing of all themes and plugins, but it will give you a starting point from some of the largest premium WordPress services.
DIYThemes — Home to the Thesis framework, DIYThemes provides theme options starting at $87.
Elegant Themes — A premium theme shop that sells individual yearly memberships for $39 per year. Developers can purchase a developers membership for slightly more.
WooThemes — Woo Themes is one of the largest theme shops. They sell theme packages starting at $70. In addition, they have plugins that are designed to work specifically with their themes. Their most recent addition to the WordPress space includes WooCommerce, ecommerce plugins for WordPress.
StudioPress — StudioPress is the author of Genesis (one of the most commonly used frameworks for WordPress). Pricing for Genesis Framework and one child theme start at $79.95.
WordPress.org — One of the best places for plugins comes from the WordPress repository. You can access the repository by going straight to http://wordpress.org/extend/plugins/ or you can also access it from within the WordPress dashboard.
“Gravity Forms” by rocketgenius — There are many great premium plugins out there, but Gravity Forms is one of my favorites. This paid plugin allows you to customize and extend online forms on a site. Check them out. They are very user friendly and can be customized. They are widely regarded as one of the best plugins in the WordPress community.
Backup Buddy — This premium plugin is a must have for backing up your WordPress website. Pricing starts at $75. This is money well spent for peace of mind.
*Please note for the purpose of transparency, some of these links are affiliate links, however, I am only an affiliate for services that I stand behind 100% and use for my own site.