You can create different types of bodies inside its physics world with different parameters like restitution, friction, velocity etc. attach them with different types of Joints like spring, rope, chain, pulley etc. Due to its native Physics engine the accuracy in solving is great.
One can visualize the motion with the numerous built in tools like tracers of points on body or Body ghosting, Graphs between different parameters( like KE, speed, velocity, momentum, etc), FBD of grouped and ungrouped objects, Camera tool ( to set frame of reference) etc.
It supports gravitational , electric, magnetic and buoyancy fields. One can even set variable fields ( time dependent ) and can easily change the fields as well using sliders.
One can create their own GUI elements in it like buttons , sliders , checkboxes , List , dialog etc. and even can write scriptable codes in them for different events in its in-built powerful scripting editing tool.
Nice interface and you can even add extra fields and script on buttons and sliders as well.
Algodoo - Algodoo is a 2D simulator freeware product designed as a physics learning tool. It was originally created by Emil Emerfeldt as part of his master’s thesis in 2008. Read more about Algodoo.
PhET Interactive Simulations - Founded in 2002 by Nobel Laureate Carl Wieman, the PhET Interactive Simulations project at the University of Colorado Boulder creates free interactive math and science simulations.
Physion - Physics Simulation Sandbox
LabsLand - LabsLand lets students access real educational laboratories through the Internet.
Crayon Physics Deluxe - Crayon Physics Deluxe
BookWidgets - BookWidgets is an excellent application that let you create engaging exercises in minutes and easily share a varied variety of beautiful and fun activities with your classroom and others.