Great question. In the world of the web its so easy to just grab things and use them so its good to see an increasing awareness that this is not always allowed within the context of copyright and intellectual property constraints.
I've got two suggestions:
- look for resources licensed via the GNU General Public License such as those at http://phet.colorado.edu/new/simulations/index.php?cat=Top_Simulations
- or resources licensed via Creative Commons such as those found in the Science and Technology section of the Connexions http://cnx.org