Cells Alive! is a site focused on cells (I know, I know, not a surprise).
Jim Sullivan has offered his entire 30 year career of capturing live cells on camera. There are some great interactive animations and some really amazing "caught on film" clips of cell reproduction.
Check out cell biology. This section contains two fill-in-the-blank type charts for animal and plant cells. You can choose whether to hoover over the animation or the list of terms at the bottom of the screen. Underneath the animation is an annotated list of for each term which can give students far more information about each part of the cell. As well, everything in the interactive section of the site is very cool.
There are rules for downloading the information but if you stay on the site (instead of downloading) you can use almost everything for free. The goal is to get you to purchase individual downloads and, if you become a heavy user of the site, it may be something to think about. Purchasing the downloads is how the creator of the site funds this project.
In a Biology classroom this could be used to introduce or help visually explain different concepts in cell reproduction. In a middle school science classroom this could be used to help study the different parts of the cell or talk about relative size of different objects (see How Big is a...?).
This is the type of site I love to find. Someone with an expertise has decided to share. Have a great time exploring the site and using this in the classroom.