Or for a graphical online UML editor (https://modeling-languages.com/web-based-modeling-tools-uml-...)?
If the former, then plantUML is my favourite. If the latter, there quite a few options but one that's easy and fast is http://www.umletino.com/
Mermaid is in the comments, not the main list, but I have been using it for a while now for our specs; it can be rendered to whatever image, but also on the client side which is great for embedding in 'living documents'.
Mermaid is in the comments, not the main list, but I have been using it for a while now for our specs; it can be rendered to whatever image, but also on the client side which is great for embedding in 'living documents'.