Our #1 go to for Science videos in my county is Discovery Education. We have a subscription and I use it all year as often as I can. It is full of videos and clips about all kinds of science topics. We have used it for everything from five senses, to plants and animals, to magnets, fire safety week, community helpers...the list could go on and on. It even has full episodes of one of our favorites, "The Magic School Bus". :) There are more than just science topics too. You can learn about nouns, addition, algebra, history, anything. They are all standards based too. You just type in a search for any topic you are teaching and you can narrow it by grade level. I don't know how much a subscription is for a school, but I can tell you it is well worth it. It is a great resource.
Another site I love to use for Science with my own personal children is www.pbskids.org. My 3 1/2 year old girls love to get on mommy's computer and go to "Sid the Science Kid". The other day I was swinging them and one said, "Mommy use more force so I can go higher!" I was impressed!
I also stumbled onto another site from PBS that is a great place for teacher resources. There are links to videos, lesson plans and interactive games. I like how you can narrow the search by grade level, topic and media type.
Science is so fun! I love teaching it, I just wish I had more time. Or if I had a class like on Sid the Science Kid with only 4 kids and every inquiry lesson all prepared and ready to go for me I might teach it more often. :)