1. Duolingo
My first impression of Duolingo was positive due to its easily useable mobile app. There is a placement test so you do not have to learn concepts or vocabulary that you may already know. There is an element of gamification; the user can earn points and rewards for completing lessons. I personally enjoyed how they have activities where you listen how to pronounce sentences and words.
2. Rosetta Stone
While I have not personally used Rosetta Stone, reviews online express the purpose of Rosetta Stone to be immersive and focused on using native speaker audio and images. Some features include stories, a native speaker reading a story, and a phrasebook (a book of common phrases in the chosen language). These are features that may not be as easily found in other resources.
3. Anki
I typically use Anki for flashcard reviews for exams, however, I believe it would also be a great tool for learning a language. A feature that make it advantageous would be using audio cards with spaced repetition to cement the knowledge. One of the helpful reasons Anki is useful for memorizing content is that it schedules cards to review within certain time frames to help remembering all content. There can be photos along with the content to help improve learning.