Anki is a free, open source flash card program that allows you to rate cards' difficulties and utilizes spaced repetition to show you cards just as you're about to forget them, thus optimizing learning efficiency.
The latest version creates a progress bar on the bottom of your Anki review screen.
As you answer questions, reach timeboxes, and clear decks, the progress bar advances. When it reaches the end, the addon scores an Anki Points habit on Habitica (will be created automatically). Additionally, a pop-up message tells you how many XP, HP, MP, and gold you earned, and notifies you if you've leveled up.
See https://github.com/eshapard/AnkiHabitica/wiki for the latest install instructions.