Header Image Slider allows you to easily build a slider of your header images.
Most Shared allows you to display, as a widget, which of your blog posts have the most social shares.
Jetpack supercharges your self-hosted WordPress site with the awesome cloud power of WordPress.com.
Theme-Check allows you to run checks on the current theme before uploading to WordPress. This plugin uses the same API and is kept more or less in sync with the WordPress.org uploader tool.
UI Labs offers experimental WordPress admin UI features with the aim of building upon and enhancing the default WordPress User Interface. These are unofficial core UI experiments – who knows what could happen?