draw.io --- A free open source online diagram drawing application

draw.io is a free and open source online diagram drawing application. Just visit https://www.draw.io and begin to draw your diagrams. There is no need for registration or login and you can import existing diagrams from or save your creations to your local drive or online storage services such as Google Drive, Github, Dropbox, One Drive and Trello. draw.io can also work as a Google Docs add-on. It supports many formats, including PNG, JPEG, PDF, SVG, VSDX (Microsoft Visio), HTML and XML.