On his website and YouTube channel Montana's 2011 Teacher of the Year Paul Anderson has uploaded more than 300 quality instructional videos like the ones about biology that are embedded below.
TED-Ed offers dozens of videos on a variety of topics in science. I created a playlist of TED-Ed videos about how the human body works. That playlist is embedded below.
Gooru is a service that aims to provide teachers and students with an extensive collection of videos, interactive displays, documents, diagrams, and quizzes for learning about topics in math and science. As a Gooru member you have access to hundreds of resources according to subject areas such as chemistry, biology, ecology, algebra, calculus, and more. Within each subject area you can look for resources according to media type such as video, interactive display, slides, text, and lesson plans. When you find resources that you want to use, drag them to the resources folder within your account. Gooru also offers you the option to add resources to your folders even if you did not find them within Gooru.
Learners TV has organized hundreds of academic videos. They've also organized more than one hundred science animations. The science animations on Learners TV are organized into three categories; biology, physics, and chemistry.
ScienceFix is the blog and YouTube channel of middle school science teacher Darren Fix. On both the blog and the YouTube channel you will find more than 100 videos demonstrating various science experiments, demonstrations, and middle school science lessons.
Bright Storm's YouTube channel offers video lessons for biology, chemistry, and physics. The videos are nothing more than an instructor lecturing with a whiteboard for a few minutes which could be adequate if a student just needs a refresher on a science topic.
NASA has a few different YouTube channels, but the one that has the most universal utility for teachers and students is NASA eClips. NASA eClips is organized according to grade level with playlists intended for elementary school, middle school, and high school.
Reactions: Everyday Science is a YouTube channel that was formerly known as Bytesize Science. I have featured a few Bytesize Science videos in the past. Reactions: Everyday Science produces short explanatory videos about the science in common elements of our lives. In the past I've featured Reactions videos about the science of snowflakes and the science of grilled cheese.
John and Hank Green's Crash Course channel on YouTube includes courses in chemistry, ecology, and biology. They're good videos, but they do go quickly so your students might have to rewind them a couple of times to catch everything.
The Spangler Effect is a YouTube channel from Steve Spangler Science. Unlike his popular Sick Science videos which are no more than short demonstrations of science experiments students and parents can do at home, The Spangler Effect videos offer longer (15 minutes or so) explanations of science experiments. The Spangler Effect videos explain the science of do-it-yourself experiments and how you can recreate those experiments at home or in your classroom.
A note about Khan Academy: I left Khan Academy off the list because it's the best known source of educational videos. Sal Khan doesn't need my help promoting his stuff.
