iCell is a free app in the iTunes store. It gives students a great 3-D visual of plant, animal, and bacteria cells. Click on an organelle and the app zooms in, giving you not only an up close view, but also a definition of the organelle. The definitions come in three reading levels: Basic, Intermediate, and Advanced. Only have one iPad/iPod/iPhone? Not to worry, you can show it under your document camera for a large group discussion.
Requires iOs 4.3 or later and is compatible with iPad, iPod Touch, and iPhone.