The Molecular Workbench is a free, open-source tool that creates and delivers visual, interactive simulations for teaching and learning science and engineering. It needs to be downloaded, rather than run through a browser, and you will need the latest version of Java installed too. But it is totally free.
Molecular Workbench allows teachers to create new activities and/or modify existing ones through its authoring system. Finished simulations simulations can be run as applets embedded into a Learning platform, blog, wiki, or other web page.
It covers a range of topics including:
- Gas Laws
- Fluid Mechanics
- Properties of Materials
- States of Matter
- Phase Change
- Heat Transfer
- Chemical Bonding
- Chemical Reactions
- The Genetic Code
- Protein Synthesis
- Electron-Matter Interactions
The software looks suitable for A Level chemists and physicists and maybe some upper GCSE students.
A level science teachers should definitely take a look. You can download it at : http://mw.concord.org/modeler/
Let me know what you think of it in the comments!
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About Me
Danny Nicholson : Educator, Science teacher, ICT Consultant, PGCE lecturer, Author and Web2.0 / SMART Masters/ Interactive Whiteboard Trainer. Contact me for any training requests. 
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