WeBWorK is a well-tested assessment system for delivering individualized problems over the web.

Whether as a student or instructor, I never really liked computerized tests.
But I had never heard about this FOSS project before. It is built on MathJax, and expanding into other STEM areas such as chemistry:

The [Open Problem Library] now includes problems from all ranges of mathematics, from pre-algebra through real analysis, as well as expanding into other STEM disciplines such as physics, chemistry and engineering.

Very nice explanation!

• 00:00 Most people don't understand rainbows
• 01:08 Light refraction explained
• 08:20 How does a rainbow form?
• 13:10 Circular rainbows
• 15:14 Why are rainbows curved?
• 17:30 Why can’t you see a rainbow with sunglasses?
• 18:43 Why is it brighter underneath a rainbow?
• 20:44 Different types of rainbow
• 23:46 Invention of the cloud chamber