Nibbler - Free tool for testing how good your website is, and what you can do to improve it. Check accessibility, SEO, social media, compliance and more.
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.
Pennywise - Cross-platform application to open any website or media in a floating window
draw.io - Online diagramming application
PiP-Tool - PiP tool is a software to use the Picture in Picture mode on Windows.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.