I guess posts have been made about where you can find references on Sindarin, Quenya, Nandorin, Old Elvish, and Noldorin among others on Tumblr but I wanted to make my own list (so I can come back to it when I need it). If you know a website you'd like me to consider for my list, please just mention it to me.
Ambar Eldaron – Sindarin and Quenya dictionaries and courses
Ardalambion – Website about the sounds of Tolkien's languages and their grammar
Automatic Quenya translator – English to Quenya and back
Eldamo – A lexicon of Tolkien's invented languages including fonts
Glæmscrafu – This website explores the sounds of Tolkien's invented languages and includes several texts and poems
Gwaith-i-Phethdain, The Fellowship of the Word-Smiths – Very detailed page by Ryszard Derdzinski. Many poems and calligraphies. Particularly noteworthy are the translations of the Elvish texts and songs from the film trilogy.
Hiswelokë's Sindarin Dictionary – A Sindarin and Noldorin dictionary that was started in 1999 and then went on hiatus in 2008
I Lam Arth – Aaron Shaw's site with a series of in-depth articles on Sindarin
Lalaith's Middle Earth Science Pages – Website about Middle-earth's people, languages of the humans, and other things. This also includes Rohirric if you're into learning that.
Mellonath Daeron – Some useful Elvish phrases
Parf Edhellen – A Sindarin, Quenya, Primitive Elvish, and Noldorin dictionary. It's my favourite website because it also compares old and new translations.
Parma Tyelpelassiva – Texts, songs, poems and grammar for Sindarin, Quenya, Andûnaic, Noldorin, etc.
Quenya pronunciation guide
RealElvish – Phrasebooks on Sindarin, Quenya, Andûnaic, etc. including naming traditions and lists for Elves, Rohirrim, Hobbits, Dwarves (if you need it for a fanfiction)
Sindarin.de – The first Sindarin website I frequented and where I learnt all the grammar. It's in German.
Sindarin phrasebook by slarmstrong – There are 3 parts of this phrasebook
Sindanórie – Site by Roman Rausch with articles on Telerin and word overlaps between the Elvish and real languages, poems, sound files, etc.
Tecendil – If you want to see what your name or others look like in Elvish script
The Dwarrow Scholar – Anything Khuzdul
Tyalie Tyelellieva – Website by Lisa Star but since geocities was taken down, it only exists in an archived form. It was last updated in 2004 but it also has a Khuzdul or Dwarvish Wordlist.
