Here are 3 websites that I highly recommend.
This site is A-MAZ-ING! The first year is free. I used my first year this school year, and it's so great that I have since talked to our principal about purchasing it for our site, to which she has agreed. You can either use their unit plans for each grade, or use it along with your curriculum (which is what I did). Basically I would "search" on their site for experiments to go along with what we were currently covering in our texts. The kids absolutely loved it and would ask every single day if we were going to do "mystery science" that day. The video is completely engaging, still allows for you to stop and teach, and nothing beats the hands on learning pieces that go with it. Best investment ever!
For transitions, lining up, class switching, etc, these songs worked miracles in my class. The tunes are catchy, and the kids become trained to the point that as soon as they hear the music start, they know just what to do. Here is two of my favorite that I used all year, and will be using again next year.
For those times when you want to print up some fun worksheets for your students for extra practice, or even for some fun during the holidays, this site has FREE worksheets (my favorite price!). I only used site for the worksheets from time to time, but they also offer guided lessons as well. A good site to have on hand.