Cobocards is a free, web-based flash card application with an emphasis on collaboration, so you can create and study a set of flash cards alongside your friends or fellow students.
We've covered flash card apps in the past, but Cobocards sets itself apart with an impressive feature set. Not only does it do all the basics, like tracking your progress as you study, but it also sports several useful features we haven't always seen from its competition, such as several printable templates, an activity feed and message board for keeping track of your friends' and collaborators' activity (like if they've added new cards to a deck), and, of course the collaboration itself.
For a fuller understanding of how it works, hit up the video demonstration on the homepage. If you've already got a preferred study companion for this sort of work, let's hear about it in the comments.