Online sequence diagrams from code. Used to be good but now lacks functionality and design. Has interactive diagrams which is cool.
Webgraphviz - WebGraphviz is Graphviz in the Browser
draw.io - Online diagramming application
Penpot - Design freedom for teams
SequenceDiagram.org - Clean and fast online sequence diagram tool using scripting and Drag and Drop
Graphviz - Graphviz is open source graph visualization software. It has several main graph layout programs.
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.