Language-related links

The Language Construction Kit by Mark Rosenfelder
A Conlanger's Thesaurus and the Swadesh lists - two good resources for choosing your language's base words
Vulgar: a Fantasy Language Generator - has a free version but the paid version is only like 20 quid and has loads of useful features
Learn NOT to speak Esperanto by Justin B Rye
Conlang syntax tests
A method of generating "flavoured" words
Omniglot - the online encylopaedia of writing systems & languages

Ardalambion - a comprehensive site on Tolkien's languages (if you're interested in Tolkien, I also recommend A Secret Vice: Tolkien on Invented Languages, edited by Dimitra Fimi & Andrew Higgins!)
Toki Pona - a conlang with extremely limited words
Oou: the insane language - "even Oou experts cannot communicate clearly using it, as miscommunication and confusion are integral to the language"
Ithkuil - a language that uses "a matrix of grammatical concepts intended to express deeper levels of human cognition more overtly, logically, and precisely than natural languages"
Hanziyu - a language based on Chinese characters
Seraphim - a multi-channel angelic conlang
Tapissary - a hieroglyphic language

Lernu - the website for learning Esperanto
sona pi toki pona - playlist of videos to learn Toki Pona
Wanikani - great system for learning Japanese kanji
Scottish Gaelic on Duolingo - yeah yeah I know about the memes but the Scottish Gaelic course is actually really good