Windows/Mac/Linux (all platforms): ANki, a free "spaced repetition system" (i.e. flashcard-style memorisation tool), offers a gentle learning curve, a pared-down software interface, and online access and synchronisation.
Once you install and launch Anki, you can easily spend hours discovering all its neat capabilities and tricks—like an HTML editor for manually designing your "cards," audio embedding, tagging, and many more—but setting up a basic "deck" and "cards" is hardly rocket science. Hit the big "plus," choose a basic deck style (or use a pre-templated style you created), and write the front (question), back (answer), and tags of your cards one after another.
Once you've created your decks, you can hit File, "Save and Sync" to make them available on other Anki installations, or access them through a free Anki account.
Check out a video introduction of Anki's features and possibilities below.
Anki is a free download for Windows, Mac, and Linux systems.