What Is It?
Needless to say, when I realized that my days of feeble attempts at drawing the human vocal tract were over, I rejoiced! This website is everything I ever hoped for and more.
Why Is It Good for ELL Students?
It can also be good for students to compare how sounds represented by the same letter in, say, English and Spanish are produced quite differently in both languages. For example, have your students watch the flash animation for how the letter /r/ is pronounced in English. Tell them to practice moving their tongues they way it is shown in the diagram. Then show your students the animation for the pronunciation of [r] in Spanish. It's totally different! This type of phonetic comparison can help make your students more aware of how they produce sounds in their native language (if the native language is Spanish or German), as well as in English.
The one complaint that some teachers have about the site is that it was created by linguists, and the letters are categorized under their linguistic names. So, if you don't know that /r/ is a liquid sound in English, it can be hard to find. Most ESL and Bilingual teachers have to take at least one linguistics course in order to become certified, so this shouldn't be totally alien to them. And if it is alien to you, well, sounds are classified according to how and where they are produced, so it can't hurt to learn some extra vocabulary while exploring the anatomy of the vocal tract. You might learn a little bit about how you speak in the process (that's my two cents, anyway - don't hate me for it).
Anyway, if you work with language learner students, I am sure that this site will be useful to you. Use it and enjoy!