By Fu-Kwun Hwang, National Taiwan University, Department of Physics.

http://www.phy.ntnu.edu.tw
/~hwang/relativeVelocity
/relativeVelocity.html

This java applet lets you view objects from different frame of reference. It is the classic river with current, boat and observer problem. The observer is on the bank of the river or can even swim across the river. Changing the frame of reference is done by moving the mouse to different regions.