Quizlet (available via web browsers and via apps for Chrome, iOS, Android, Kindle Fire, Nook, and Windows phones). This post explores the web or Chrome app version.
Quizlet, students and teachers can sign up using their CCSD GoogleApps logins if they will be creating sets.
There are 6 study modes for sets. Not only can you study using the flashcard mode, but you can also have it generate practice for you, test your spelling, create assessments and games. Immediate feedback is built into the modes.
Quizlet also has image, audio, and multiple language support. When creating flashcard sets, you can include images in the definition that match your term (it has a built-in image search tool or you can upgrade your account and upload your own images). You can also choose different language options (useful for world language teachers). Finally, when clicking on terms or definitions, Quizlet will read the terms or definitions out loud for you (try it out in the embedded example below).
Speaking of embedding, Quizlet provides embed code so that you can put sets into other sites (like Schoology or blogs). That way, students can use sets without having to log in or search Quizlet's site. A pull-down menu in the lower corner lets you choose between study modes if the set has been embedded (the example below is for the 5th Grade FOSS unit for Mixtures and Solutions).
When creating card sets, you can also combine with other sets, create folders, invite others to edit, or make them private. As a teacher, you can also create a class for internal collaboration.
If you or your students would like to use flashcards on any device for free, Quizlet is worth a look.