A teacher's website with many printables. There are also tools for making your own worksheets. Some are available for free, but a subscription gives access to more tools and interactive resources as well. (Fortunately, it's pretty easy to see what you can get for free and what you need a subscription for.)
Free art lessons for elementary ages.
Online reading, math, and spelling curriculum for elementary ages.
Complete math curriculum for jr. high and high school levels.
This subscription site has loads of resources for learning through children's literature.
A website with lesson plans, worksheets, labs, and activities on biology topics.
The website of this museum in Boston has lots of informative articles, videos, and some free less