and then there is always the ubiquitous jquery for reducing the amount of browser stupidness - http://jquery.com/
And if you want to do apps, you could do worse than trying out unity3d - http://unity3d.com/ (It has a free version for learning it, though it will cost you a lot to do proper commercial work with it)
three.js - http://mrdoob.github.com/three.js/
kinetic.js - http://kineticjs.com/
d3.js - http://d3js.org/
and then there is always the ubiquitous jquery for reducing the amount of browser stupidness - http://jquery.com/
And if you want to do apps, you could do worse than trying out unity3d - http://unity3d.com/ (It has a free version for learning it, though it will cost you a lot to do proper commercial work with it)