• Omniglot is an impressive guide to over 500 languages, including detailed information on over 180 different alphabets, syllabaries and other writing systems, with plenty of background information and written samples. It also includes useful phrases in a large number of languages, many with audio.
• alphaDictionary also has many links to grammars and language courses, as well as dictionaries; based upon yourDictionary.com.
  • iLoveLanguages was an encyclopaedic set of links for a broad range of languages, but seems to have been neglected lately.
• Open Culture has pages of links to free online courses. 
• Loecsen Essentials have a good collection of basic words and phrases in over 40 languages, represented pictorially, with a quiz.
• UCLA Language Materials Project has a database of bibliographic references for over 900 languages, which can be searched by type of material, level of instruction, whether audio is available etc., often with methodological information. There are also detailed profiles of over 100 of the languages.
• Routledge's Colloquial series - good quality book+audio courses in 70 or so languages, with the audio component freely available on their website.
• Live Lingua Project collects together major public domain language learning materials, including the FSI Language Courses in many languages, developed by the US Department of State's Foreign Service Institute in the 1960s and 70s.
  
  •  So you want to learn a language links to many similar materials, especially from the e-books page.
• Less Commonly Taught Languages project, based at the University of Minnesota, focuses on where you can go for courses in your chosen language, though it also has some interesting royalty-free instructional materials.
• SOAS Language Centre offers evening classes in many African and Asian languages.
• Languages of the Wider World CETL - Centre for Excellence in Teaching and Learning, based at SOAS and UCL.
  • They have a number of high quality online resources in a variety of languages.
  • ATLAS offers a good taster of many languages: Arabic, Czech, Danish, Dutch, Greek, Hebrew, Hindi, Lithuania, Mandarin, Polish, Portuguese, Russian, Swedish, Tigrinya, Turkish, Urdu and Yorùbá.
• The CEELBAS Language Repository has open-access teaching and self-study materials for languages of Central and Eastern Europe and Russia, some specifically aimed at social sciences/humanities researchers. Languages include Bulgarian, Croatian, Czech, Estonian, Finnish, Georgian, Hungarian, Polish, Romanian, Russian, Serbian, Slovak and Ukrainian.
• National Council of Less Commonly Taught Languages (USA).
• UNESCO Atlas of the World's Languages in Danger, part of their Endangered Languages project (presented for 2009 International Mother Language Day, 21 February).
  • See also Cambridge's World Oral Literature Project.

In roughly alphabetical order of language...

• hayya bina (Let's go!) is a web-based course of Modern Standard Arabic for beginners, developed with EU funding by partners including the University of Exeter. Learners can study with Western script, with Arabic script, or with both; it's clearly presented, with plenty of audio (recording too), and users can even choose to study in different mother languages.
• Madinah Arabic offers a reading course as well as a spoken language course.
• Babel:arabic - basic lessons with sound, plus grammar lessons.
• Scripts of All Asia shows characters and their corresponding sounds in many Brahmi, East and Southeast Asian languages.
• Beginning Bambara (An Ka Bamanankan Kan Kalan), the major language of Mali - audio lessons from Indiana University, who also offer an intermediate course. See also Bambara 101 which takes a text-based approach.
• Larry Trask's Basque Page - material by the late University of Sussex professor of Linguistics has been thoughtfully preserved at Buber's Basque Page which includes language-learning material and cultural background.
• Survival Bengali provides an introduction to the language via a few lessons, phrases with audio and a small dictionary (from Virtual Bangladesh).
• BSL (British Sign Language) SignBank video dictionary from UCL.
• Learn Bulgarian
• Intercat is a collection of Catalan-learning resources from nine Catalan universities.
• Cheyenne
• Chichewa vocabulary and sentence drills (a Bantu language spoken primarily in Malawi and parts of Zambia and Mozambique).
• Help Me Learn Church Slavonic - the liturgical language of the Russian Orthodox Church. There's more at the University of Texas and also Pravoslavnaia stranitsa (in Russian).
• Croatian Language - basic phrases.
• LearnOasis have a basic course in Danish (as well as Arabic, Turkish, Hungarian and Spanish.
• Taalthuis - some free multimedia lessons on the Dutch language, at beginner and intermediate levels.
• lernu! - several online courses in the international language Esperanto. 
• Finnish language
• On-line Gaelic at the University of the Highlands and Islands has hundreds of classified links to pages in and about Scottish Gaelic and other related languages - there are plenty of links for Gaeilge (Irish Gaelic) there too.
• Gaelic Homepage with lots of resources and links to further information on the three Gaelic languages.
• Irish Gaelic course online with sounds and exercises.
• galego.org - articles on the Galician language: its history, sociolinguistics, a small dictionary etc. In Galician, Spanish and English.
• Introduction to Modern Greek leads you gently through learning the alphabet, numbers and basic grammar - includes audio and doesn't need Greek fonts installed.
• Greek through the Internet - a series of lessons and drills, plus grammar and syntax to help you learn Modern Greek.
• Hausa CALL is a multimedia introduction from SOAS, with exercises.
  
  • See also tutorials and references from Russell Schuh at UCLA.
• FoundationStone Online Hebrew Tutorial
• Hungarian<->English Dictionary
• Icelandic Online
• Languages and Scripts of India is particularly strong on Hindibut provides an excellent starting point.
  • Devanagari Script Tutor is a Flash-based tutorial (with tests) developed for SOAS.
  • Major Indian Languages including links to further online resources.
  • See also Urdu below.
• Indonesian Listening Materials - audio with transcripts and worksheets, on many topics at a range of levels.
• Introduction to Korean - an excellent set of reassuringly easy-to-follow tutorials for beginners - includes audio and doesn't need Korean fonts installed.
• Korean in the University Library - a number of e-books are available to University of Sussex members, for example Elementary Korean with accompanying audio freely available on the publisher's website.
• Cambridge Latin Course website accompanies the books in the Language Learning Centre with online activities exploring the stories and vocabulary, a dictionary with audio, vocab tests and even web versions of the textbooks.
• London Latin Course on YouTube - a structured, slow-paced series of videos from the ground up. 
• Medieval Latin - a collection of resources from Georgetown University.
• Dictionary of Louisiana Creole edited by our own Christophe Landry. 
• Talk Portuguese from the BBC in 11 bitesize videos; Centro Virtual Camões goes into a bit more depth. Brazilian Portuguese is rather different.
• Culture of the Andes - Quechua songs, stories, poems and jokes, with translations in Spanish and English. Includes audio pronunciation.
• Romanian Listening Materials with transcripts and slideshows, from the Languages of the Wider World CETL.
• San languages - an audio sample of Ju|'hoansi, one of the 'click languages' of Southern Africa.
• Thezaurus - Slovenian language and other resources.
• The Internet Living Swahili Dictionary with pronunciation and grammar guides.
• Kiswahili Kwa Kitendo - online audio for a coursebook in Sussex's Language Learning Centre.
• Introduction to Swedish plus English<->Swedish dictionary.
• Thai-language.com includes over 10,000 audio clips for learning Thai, alongside a bilingual dictionary of around 30,000 terms and a forum. The FAQ section is also full of tips for further study and using Thai on your computer.
• Learn Tibetan
• Learning Practical Turkish offers an entertaining way to learn the language, with "Sounds of Specialty Turkish": business, computing and culinary terms, for example, and even tongue-twisters in RealAudio!
• Read Ukrainian - well-designed supplementary materials for reading courses in Ukrainian.
• Urdu language-learning resources
• Learning Vietnamese Online
• Welsh Course from Cardiff University (though originally from Brown University), including dictionaries, a spellchecker and links to lots of other Welsh resources around the Internet.
• Wolof online - comprehensive interactive course with samples and conversation snippets.
• The Virtual Shtetl - Yiddish Language and Culture.
• Virtual Zulu from the LWW-CETL is a proper 14-lesson course with audio.