Apart from the well-known ones, such as Duolingo, I always recommend everyone to use Anki (https://apps.ankiweb.net/). I consider it the best free tool for learning any language, not just Spanish.

You learn without realising it, through flash cards and a methodology based on intelligent repetition.

Here is a list of decks, most of them free, to learn Spanish: https://ankiweb.net/shared/decks/spanish