Doing some research on HTML5/Canvas usage I found theses tools on the web:

• http://pencil.evolus.vn/en-US/Home.aspx Pencil Project: graphic editor, nice replacement for dia
• http://www.websequencediagrams.com/ Web Sequence Diagrams: simple and usefull UML sequence diagrams tool, I also like the http://zwibbler.com/ Human Drawing editor from the same author, nice code.