I'll put a handful of links here to start off
https://www.livelingua.com/fsi/
http://www.bbc.co.uk/languages/
https://apps.ankiweb.net/ (flash cards)
https://www.duolingo.com/ (this ones controversial, but I believe it's good as long as you pair it with other resources)
https://wikitongues.org/
https://en.wiktionary.org/
https://lang-8.com/
Post any other resources you all know (pdfs, videos, podcasts, courses, lectures, etc.) either specific to certain languages or more general.
(Any resources relating to learning Mandarin and Arabic would be greatly appreciated by OP)