A single mass attached to a spring, which is connected to a wall. An example of a simple linear oscillator. 

• You can change mass, spring stiffness, and friction (damping).
• You can drag the mass with your mouse to change the starting position.
• Energy can be observed (potential, kinetic, total) by clicking the "show energy" checkbox. 
• Graph tab shows any two variables including energy.
• Time Graph tab shows any three variables against time.

Explanation of the math is available, along with open source code.