Magnific Popup - The greater part of Lightbox modules will completely preload a picture before showing it.
draw.io - Online diagramming application
jQuery Lightbox - jQuery Lightbox plugins are utilized to showcase images and other web stuff using modal dialogs.
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
Fancybox - fancyapps.com - Fancy App Store
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.