If you find yourself looking for more resources on Math, History or ELL, your best chance is to check out blogs or feeds dedicated to these topics. A lot of them include video archives, tech tools and tutorials for them, and pointers for effective instruction. The most important characteristic, though, is that most of these are written by actual teachers in the field. These were borrowed from Richard Byrne's Free Technology for Teachers blog.
Mathematics and Multimedia written by Guillermo Bautista
Dan Meyer's blog on challenging the way that math instruction is delivered
Numberphile,YouTube channel about fun number facts, rules of mathematics, and the ways that our brains handle numbers.
ELL and ELA:
Larry Ferlazzo Blog for sharing websites that will help you teach ELL, ESL, and EFL
Life Feast written by Ana Marie Menezes, for teachers interested in using technology in elementary and middle school ELL/EFL lessons.
English Companion, by Jeff Burke, is more of a website of excellent resources than it is a blog.
Teaching the Civil War With Technology, written by Jim Beeghley
Social Studies Central which houses a good collection of resources for social studies teachers.
US National Archives is an all around good resource for history teachers
Today in History from The Library of Congress offers a daily artifact feed similar to the one offered by the National Archives. It includes a new image or document along with the story of the notable event or person connected to it.