Raphael seems a pretty good canvas library; it's SVG-based (or VML-based in Internet Explorer), and thus supports a lot of user input events. It's fairly small (60kb gzipped), so isn't too large a dependency.
It seems to have a nice tweener too: http://raphaeljs.com/reference.html#animate (see here and here for examples).
For an example of what it can do, have a look at this clever little demo.
Hope this helps!
