BrainRush is an excellent new site that I just found out about from Larry Ferlazzo's blog. BR is a fantastic site for students to create and learn through flashcard style games. Also, these games utilize a "adaptive practice technology" which is ideal for Differentiated Instruction as it tailors learning to each individual student needs.
Cram is a nice site that I just found out about from Free Technology 4 Teachers for making flashcards. This is a site where teachers can browse through a wide variety of flashcards on any number of subjects as well as create their own. Also, this is a nice place to find a list of nice mobile apps for creating flashcards.
Brainscape is a wonderful site/app that allows teachers or students to create flashcards. These flashcards can be created online and then shared to any iOS mobile device. Best of all these cards can: contain images/audio, can be shared, are ideal for mobile learning, and can be used by any subject.