Agreed aizen!

The applets here are really helpful in learning.

The feature that is cool for me is:
the centre of mass., will add it here Ejs open source java applet 1D collision carts Elastic and Inelastic Collision http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=831.0 when time permits!

Thanks Prof Fu-Kwun Hwang!