👨💻 Interactive Courses
These are mobile apps and websites in which you take turns reading explanations and going through interactive activies.
Bite-sized learning
Duolingo is the lowest friction way to get started. It breaks down learning into a daily, 5-minute-long gamified session (or more if you want). It's very low on explicit explanations, especially grammar, so you should pick a more complete course soon to complement it. You can use it via web or mobile apps, and you can complete the entire course in their free plan. You can start using it without an account, but you can create one later if you want it to track your progress across devices.
Introductory courses
I Kinda Like Languages has a conversational style, introduces concepts step by step, suggests mnemonics to help remember things, and exercises come as small and frequent interactions. Mobile-friendly, no account required and entirely free.
Yakk follows an interesting concept trying to combine the depth of a book with the interactivity of apps. Written in a very engaging style and peppered with media and activies. Mobile friendly, requires an e-mail address but the Esperanto course is free.
Esperanto in 12 Lessons focuses on the 500 most commonly needed words in conversations (the Zagreb method). Each lesson includes a text (with clickable words for explanations), a grammar clarification and 3 exercises. Available in many languages, also free and mobile-friendly, no sign up required.
Full courses
Lernu! is one the oldest, most established and well-known Esperanto online course. You learn by following the story "La Teorio Nakamura". It has solid grammar explanations, and in the same website you can access many integrated learning tools like a complete grammar, a dictionary and a library of learning media. Available in many languages, requires sign up but is entirely free.
Kurso de Esperanto Kape is another popular and well-regarded course. It has detailed explanations and is very audio rich, you can click almost any word and hear a custom recording of its pronunciation. It also uses a few songs for lessons. Available for Android and desktop platforms, in many languages. Completely free and no account required.