Chemistry
OrgChem101 is a nice site from the University of Ottawa, with interactive modules teaching organic chemistry concepts. It also has a French version
MIT OpenCourseWare: Introduction to Solid State Chemistry
Washington University and the Howard Hughes Medical Institute have created an online tutorial to help prepare students for college-level chemistry courses. http://www.chemistry.wustl.edu/~coursedev/Online%20tutorials/chemtutorials.htm
I think the online course movement is wonderful. Here, for example, is MIT’s offering on the basic principles behind chemical science. Watch the video lectures, work along with the lecture notes (filling in the deliberately-left blanks), read the readings, and test yourself on the exams.
Chemistry Educational Digital Library http://www.chemeddl.org/
Something a little different -- the University of Nottingham's innovative series of short videos about each element in The Periodic Table of Videos
http://www.periodicvideos.com
General Chemistry Online has a lot of resources for the chemistry student
https://antoine.frostburg.edu/chem/senese/101/index.shtml
If you need to know about organic chemistry, check out the interactive 3D animations of important organic reactions at https://www.chemtube3d.com/
The periodic table on the web
http://www.webelements.com/
Periodic Table Memory Pegs
http://www.johnpratt.com/atomic/periodic.html
Interactive Periodic Table
http://environmentalchemistry.com/yogi/periodic/?new=periodic/
Periodic Table Challenge
http://www.ilpi.com/genchem/periodicquiz.html