Helen Hou-Sandi

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.
<h4>Features</h4>
<ul>
<li>Add as many widgets as you want, wherever you want</li>
<li>Display images from a specific post/page, the entire Media Library, or a specific set of attachment IDs</li>
<li>Display images from the post/page currently being viewed</li>
<li>Select a number of images</li>
<li>Select any registered size in WordPress</li>
<li>Display the images in various orders</li>
<li>Show or hide captions</li>
<li>Link the images to the original file, post, anchor in the post, attachment page, or nothing. NOTE: If the image is not attached to a post, the file link will be used instead of the post or anchor in the post.</li>
<li>Add a prefix to the link and image title (appears as a tooltip)</li>
<li>Use a rel attribute for the link – great for lightboxes</li>
</ul>

Really Simple Gallery Widget

Simple widget for displaying images from the Media Library with plenty of configurable options.

Aaron D. Campbell

A widget that displays rotating images from a post’s gallery in the sidebar of 
your site. You just specify the post or page, and it will cycle through all the 
images from the gallery of that post or page. you can have as many of them as 
you want on your site simultaneously. Requires PHP5.
In WordPress 2.6, another plugin of mine has been included into the CORE to 
allow these to be reorder your images to make them display in whatever order you 
prefer. Until then, go get <a href="https://wordpress.org/extend/plugins/reorder-gallery/" rel="ugc">Reorder Gallery</a>!

Gallery Widget Pro

A widget that displays rotating images from a post's gallery in the sidebar of your site. Requires PHP5.

Really Simple Gallery Widget vs Gallery Widget Pro