Webhook.site - Instantly generate a free, unique URL and email address to test, inspect, and automate (with a visual workflow editor and scripts) incoming HTTP requests and emails.
PhotoSwipe - JavaScript gallery, no dependencies.
MockServer - Easy mocking of any system you integrate with via HTTP or HTTPS.
Magnific Popup - The greater part of Lightbox modules will completely preload a picture before showing it.
Beeceptor - Unblock yourself from API dependencies, and build & integrate with APIs fast. Beeceptor helps you build a mock Rest API in a few seconds.
Fancybox - fancyapps.com - Fancy App Store