Label 59 is a neat web app that allows you to create labeled slide shows and illustrations. After playing around with it, I can say there is a bit of a learning curve but the controls are minimal and intuitive. Middle to high school students could easily use Label 59 to create illustrations of scientific processes or label dissections. Geographic features could be labeled or even a step-by-step process outlined. Teachers could use it to create custom illustration or demonstrations.
If you plan to use Label 59 in the classroom, plan to spend time training students how to use the different features. They will also need an email address to sign up for a free account. Also plan on how students will share their final results.