To show my appreciation for Duolingo, I've started to compile a list of adjectives that describe Duolingo:

Addicting, cutting-edge, educational, eminent, enjoyable, engaging, enlightening, enriching, entertaining, enthralling, enticing, excellent, fantastic, fascinating, first-rate, free of charge, impressive, intriguing, innovative, inspiring, interactive, intuitive, marvellous, novel, outstanding, revolutionary, stellar, straightforward, superior

Please comment below if you think of any more, and I will add them to the list.