Selected plugins in the top 30

Other plugins in the top 30 that get an honourable mention are

TitleExcerpt
WP Super Cache & W3 Total CacheWP Super Cache and W3 Total Cache both try to make your WordPress site faster.
WooCommerce and WP-e-commerceWooCommerce - extendable eCommerce plugin, from WooThemes, WP-e-Commerce
BackWPUp and BackUpWordPressA couple of choices for performing a backup of your WordPress site. Just in case anything nasty happens.