This is a crazy collection and categorization of mathematical oddities.
http://www.recmath.org/PolyCur/p6odd/index.html
Here, the word oddity has a specific, technical mathematical meaning: an oddity is a figure with at least one line of symmetry that is made up of an odd number of copies of some polygon.
I’ve never heard of oddities before, and I learned a lot of new vocabulary words while exploring this page: polyform, hexapent, and quinquerotary, to name a few!
This is an amazing resource: a collection of science demonstrations from Harvard Natural Sciences Lecture Demonstration Services.
https://sciencedemonstrations.fas.harvard.edu/catalog
There are hundreds of demonstrations in the archive. Each entry includes an in-depth description of the scientific principle(s) on display in the demonstration, as well as detailed instructions on how to execute the demo.
Topics include Newtonian Mechanics, Light and Optics, and Thermal Physics. There is also a collection of mathematical demonstrations, as well as a small set of videos.
And be sure to check out their Facebook page here: http://www.facebook.com/NatSciDemos
This is a great resource: a brief history of many mathematical symbols.
http://jeff560.tripod.com/mathsym.html
Ever wonder where the symbol for 
There’s all that, and much more! Although, I would like to know more about the bar used to represent repeating decimals.
This is a great resource: an archive of Problems of the Week from MathCounts.org.
https://mathcounts.org/Page.aspx?pid=1573
MathCounts is a national math enrichment organization that, among other endeavors, runs a middle school math competition every year. Their “Problem of the Week” is typically aimed at middle school students, but these problems may also be of interest to high school students and teachers.
There are over 10 years of math problems in the above archive. The problems range in scope from basic “Math Team” style questions to thematic questions based on current events, dates, and holidays.
This is a collection of free online college courses from the Indian Institute of Technology — Delhi.
http://nptel.iitm.ac.in/courses.php
There are a wide range science and technology courses represented here: Electronics, Mechanics, Information Systems, as well as several Mathematics courses.
There is a mixed-bag of resources available here: many of the Basic Courses have a full curriculum of lecture videos, while some of the more advanced courses currently offer only on-line syllabi.
Another nice resource for students and teachers!