Bjørn Johansen

This is a WordPressified version of the excellent, lightweight (for real!) imageLightbox.js (MIT-licensed) by Osvaldas Valutis (http://osvaldas.info/image-lightbox-responsive-touch-friendly)
This is his introduction to the lightbox: 
“As for the image lightboxes, I have always lacked simplicity for them. I mean not only the visual design, but the overall experience: from implementation to UX. All due respect, but I have never liked any of these light-fancy-face-boxes. They all were solving problems that just don’t exist: lots of unwanted features by default, total mess in markup, no native behavior for touch screens. I was working on a project and needed an image lightbox that solves these problems. I needed a lightbox only for images, not video, text, and iframes at once. I made ImageLightbox.js.”
This version autodetects all links to image files and opens them in the lightbox.
For your “install, activate and go”-convenience, this plugin comes with a minimal CSS that easily can be deactivated and/or replaced.
The plugin can also be themed to your liking. An example theme is available here: https://github.com/bjornjohansen/imagelightbox-theme-smooth
Please let me know if you have any issues. Fastest way to get a response is by Twitter: https://twitter.com/bjornjohansen

imageLightbox

Image Lightbox, Responsive and Touch‑friendly.

stevegibbs

Gallery Helper is a lightweight and unobtrusive lightbox/slideshow add-on to WordPress’s image gallery block.
<h3>Usage</h3>
<ul>
<li>Add a gallery block to your page, post or cpt like usual.</li>
<li>Check “Enable Lightbox”” in the gallery block settings.</li>
<li>Optional: choose automatic slide advancing.</li>
<li>That’s it</li>
</ul>

Steve's Gallery Helper

Gallery Helper is a lightweight and unobtrusive lightbox/slideshow add-on to Wordpress's image gallery block.

imageLightbox vs Steve's Gallery Helper