The online resource I use most often is BrainPOP and BrainPOP Jr. (You actually have to have a subscription to use these sites.) This site has AWESOME videos that each come with a quiz (online or print-out), vocabulary words, and several activities (including extensions). BrainPOP is geared toward the older grades (4th and up) and BrainPOP Jr. is for the lower grades. However, I use BrainPOP Jr. for my 4th graders sometimes because the terminology is a little easier for them to follow.
Gamequarium has a bunch of games (for ALL subjects). However, the quality is kind of a hit and miss...
I just came across this one and I really like it! www.engagingscience.org has a few games that are narrated by the robot you see on the screen. The graphics are easy on the eye and the content is fairly appropriate for my students. The link is below.
Who could ever tire of this guy??! I l-o-v-e his videos and my kids are always excited to see his videos, even though the ones we watch are like, 20 years old. Bill Nye's videos have a way of explaining science (and even math) principles in an entertaining and simple fashion. I heard that he has a new series, but I haven't seen them yet.