by Helen Hou-Sandi on WordPress.org
Simple widget for displaying images from the Media Library with plenty of configurable options.

Widget options
Really Simple Gallery Widget adds a widget to display images from the Media Library, no extra uploading or creating custom post types required. You can choose to show images from a specific post/page, the entire media library, or a specific set of attachment IDs. On single pages/posts, you can also show only images attached to the item currently being viewed. Especially helpful if your galleries are based on what’s attached to a post and you want to be able to easily display those images in a widget area.