DNA Structure Tutorial
A classical tutorial (1996-) originally designed by Eric Martz (Univ. of Massachusetts-Amherst, USA) and further developed by Angel Herráez (Univ of Alcalá, Spain).
Features: 3D models of the double helix structure. Interactive, animated, nonlinear.
Level: basic to medium (14 year old onwards).
Software: just the web browser. Java is optional. Compatible with mobile platforms.
Available in Chinese, English, French, German, Greek, Italian, Polish, Portuguese, Romanian, Spanish, Thai, Turkish
Licence: 
Creative Commons Attribution-NonCommercial-ShareAlike