StudyBlue is a neat web app that lets you create digital flash cards directly from notes you take in class or anywhere else. You can also take the cards you create just about anywhere with mobile apps for iPhone and Android.
StudyBlue is free, but it seems you have to go to school to use it. If your school hasn't been added to their list of schools you'll have to wait a bit for it to be approved. This seems to be to your benefit because StudyBlue lets you add specific classes, but it's kind of annoying as you have to wait a day if your school isn't on the list. Nonetheless, once you're in you can start taking notes and making flash cards from those notes. If you're an Evernote user, StudyBlue now integrates with Evernote so you can keep your notes there and just sync them when you're ready to make your flash cards. StudyBlue also tracks correct and incorrect answers when you're studying with your cards to help you better learn the stuff you don't know.
If you're studying for finals and need some help you can take anywhere, StudyBlue is a great (and free) option.