Kids Know It Network : kid friendly science songs
I've played some of these songs for my class before, and they enjoy it. My personal favorite is called Sir Isaac Newton for teaching about forces and motion. The songs are appropriate for all of the elementary grades and cover topics from astronomy to physics, and even non-science topics such as language arts and math.
Gamequarium : science videos (like Bill Nye!)
This website has free videos on a very wide range of science topics. There are also full length Bill Nye videos! It does not require a sign up, has no ads, and is completely free to watch. Longer videos, such as Bill Nye, are broken up into 3 parts, but it's easy to go from one to the other, it gives you a chance to question students on what they've learned.
Bitesize : science videos and games
This website is produced by the BBC for all grade levels and subjects. Of course I use the science section the most. You can select a topic, read a passage, play a game, and take a quiz. I've done this on interactive whiteboards, and even in the computer lab as part of science tutoring. The games are pretty fun. I just spent about 30 minutes playing some with my husband. :)