I've unintentionally come across this website several times (today I had a need to reference a double-slit experiment), and each time it has sucked me into meandering through its excellent interactive demonstrations. While this is high-school level physics, if you haven't brushed up in a while it can be eye-opening and educational. The site appears to have been designed some time back, and aesthetically it is subjectively rather dated, but it is pure genius the way it uses interactive elements for education - This is the sort of thing that demonstrates computer and web learning at its finest.
 
Irony (or is it?) - Mental confusion over how to spell genius ("Genious? Hrmmm...")