My goal for 2020 is to improve my Spanish, and I intend to use the open educational resources described below to help me along my long path towards fluency. These suggestions are by no means an exhaustive list of resources for learning Spanish. The items included are all open resources, which means they are either shared under an open license or are in the public domain.
You should also consider using resources that are not open but are free, such as books in Spanish from your local library, videos on YouTube and similar sites about topics that interest you or that are designed for people learning Spanish, and anything else you can access that will increase your exposure to the Spanish language.
I hope these resources are beneficial to you. ¡Buena suerte!
There are a lot of books out there with the title Beginning Spanish, but in this case, I am referring to the textbooks written by Jared Reynolds and available on OER Commons. The Beginning Spanish 101, Beginning Spanish 102, and Beginning Spanish 102 TPRS (Teaching Proficiency through Reading and Storytelling) books were written by Reynolds to use as textbooks in his classes, but they are approachable beginner texts even for people studying Spanish on their own. Beginning Spanish 101 and Beginning Spanish 102 TPRS are released under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 license. Beginning Spanish 102 is released under the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 license.
Español Abierto is a collection of open education resources for learning Spanish maintained by the Center for Open Educational Resources and Language Learning at the University of Texas at Austin. The collection contains a large selection of learning materials designed for various education levels. There are resources for learning vocabulary, grammar, and more. For example, you can improve your listening comprehension by listening to Spanish speakers in videos from the SpinTX Video Archive or dive deep into Don Quixote with Recorridos. The licenses for the various resources vary, but they are all available under one of the Creative Commons licenses.
Flashcards with Anki
Anki is an open source, cross-platform flashcard program. You can use Anki to develop a personalized collection of vocabulary flashcards and study them at your leisure on your computer or mobile phone. Anki uses the spaced repetition method, which means that the flashcards you have problems with are repeated more frequently than flashcards that you have mastered. Using an optional AnkiWeb account, it is possible to sync your flashcard collection between multiple computers and mobile devices. Anki is packaged for all the major Linux distributions or can be downloaded for Windows and macOS from the Anki website. The Android port, AnkiDroid, is available from the Play Store and from the F-Droid repository.
LibriVox Spanish audiobooks
LibriVox produces audio versions of public domain books. The selection of Spanish audiobooks available from LibriVox is large and diverse. There are Spanish language audiobooks about many subjects, including books for children, which are ideal for those learning Spanish. Start by listening to something reasonable, simple, and familiar, like Jacob and Wilhelm Grimms' collection of fairy tales or other works for children, and progress towards Spanish translations of classic novels, such as the works of Jules Verne and Spanish language classics like Don Quixote.
Project Gutenberg books in Spanish
Project Gutenberg's extensive selection of public domain books includes a large collection of Spanish language classics and classic works from other languages translated into Spanish. The Spanish language collection on Project Gutenberg provides reading material suitable for intermediate learners. There are books in many different genres and about many different subjects. Project Gutenberg eBooks can be read on just about any digital device; just be careful to not become too reliant on using a device's built-in translation tools and dictionary applications to look up unfamiliar words. Try to understand things from context instead of stopping and looking up something whenever you come across a word or phrase you do not understand. Making note of a word you do not understand is fine, but rushing off to Google Translate constantly will have a negative impact on your learning.
Wikibooks' Spanish textbook
Wikibooks, which is a part of the Wikimedia Foundation family of projects, is a collection of open content textbooks. The site's Spanish textbook is a collection of nine lessons aimed at beginning learners. It is not the most comprehensive textbook, but the lessons do provide a good introduction to Spanish grammar. This textbook teaches Spanish as it is spoken in Spain, so if you are looking to learn to use "vosotros," which many North American textbooks skip because they teach Latin American Spanish, the Spanish Wikibook is a good supplement to other texts.
These are the open educational resources I plan to use to improve my Spanish. If you have favorite open resources that I did not include on this list, please share them in the comments.