Free online flashcards for HSK learners
StudyBlue for HSK learners is a free service for creating, studying and sharing flashcards.
Tools for institutions + individuals
This service is geared towards educational institutions but can be used by anyone at all, you just have to sign up for an account.
Nice apps, too
One of the strengths of StudyBlue is the incredible apps they have developed for major mobile platforms including iOS and Android. These apps are elegant, feature-rich and free!
Use existing cards or make new ones
The link above takes you to the current list of HSK flashcard sets that have been created already by users. You can either take advantage of the sets that are applicable to your study level or jump in and make custom sets for your needs.
Syncs with Evernote
If you are an Evernote user like me, you'll be glad to know you can sync your Evernote and StudyBlue accounts - very cool!