So I was remarking to a coworker today that the HTML Canvas API is very low level and hard to use, and his reaction to that was actually positive (and he had a point). By being very low level, it means that pretty much everything is exposed and going forward you won't have to wait for browser vendors to update their libraries in order to get the latest features. Basically, the browser vendors are removing themselves from the equation.
However, application developers are left with a bit of a dilemma. Do we really want to reinvent the wheel every time we build an app? Why is it that you have to redraw everything every frame? Wouldn't it be easier to work with objects rather than pushing pixels?
Well, while the browser vendors might have removed themselves from the library equation, fortunately a number of other people are stepping in. It looks like there are a myriad of 3rd party libraries out there now for manipulating the HTML 5 Canvas, some of which are more sophisticated than others.
I haven't had time to delve into any of them for real yet, but from what I have seen so far, these seem to be the most advanced (as far as being Adobe Flash replacements).
CreateJs - http://www.createjs.com/#!/CreateJS
This looks to be the most complete library out of all the ones I have seen with support not only for Canvas, but also tweening, sounds, and preloading. It also supports a stage object. EaselJs is the part that manipulates the canvas.
oCanvas - http://ocanvas.org/
This also looks good, and the code samples I have seen look very similar to ActionScript. The "o" in oCanvas stands for Object.
Paper.js - http://paperjs.org/
There are a myriad more as well. The only problem with having so many libraries out there is that you need to make sure you choose the right one when you start your project. I am not sure how inter-operable they all are and what would happen to you if you chose the wrong library.