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.
• http://typeface.neocracy.org/ Typeface, http://cufon.shoqolate.com Cufon, http://www.netzgesta.de/dev/text/ StrokeText : 3 libraries used to add embeded font support on a page (and for other js usage).
• http://www.benjoffe.com/code/all demos there, I especially like the '3D walker'
• http://code.google.com/p/iecanvas/, http://code.google.com/p/explorercanvas/ some libs to try adding canvas support in IE