Disclaimer: I’m the author of Fabric.js.
I would say that Easel.js, Fabric.js, and Paper.js are among the most used at the moment. I’m judging by the number of Github watchers for each repository, volume of discussion in their Google Groups, and how often I hear about them used as canvas libraries on Twitter.
These are also the ones with more or less decent documentation, examples/demos, discussion groups, and unit tests (the state of testing in most other canvas libraries is pretty sad).
I’m also maintaining this comparison table of various canvas libraries, where you can see how recently the library was updated, its size, support for IE<9 or node.js, and more.