There are thousands of dictionaries in the world, often by publishers trying to get onto the wagon of bookcase necessities. However, for each language or country there is often a single dictionary that is considered authoritative, either by popular consensus or by legislation.
Yes, legislation, this may come as a shock to people who only speak english, but there are languages that have legally mandated prescriptive dictionaries often governed by some official language institute.
This is in contrast to English which only has descriptive dictionaries. These English dictionaries describe the language as it is found by researchers, and in the spelling in which it is found, without any official rules.
The following is a list of dictionaries considered as authoratitative in their language or region (in random order).
Van Dale Groot woordenboek van de Nederlandse taal - Van Dale Great dictionary of the Dutch language
There is an official government spelling guideline and word list. However this "groene boekje" does not contain any meanings of words, only their spelling. The commercially published "Van Dale" is the undisputed authoritative dictionary for meanings in Dutch.
Le Dictionnaire de L'Academie Francaise - The Dictionary of the French Academy
This is the official dictionary of the French language as mandated in France
Uniwersalny słownik języka polskiego PWN - The Universal Dictionary of the Polish Language
This is not a legally mandated dictionary, but it is the standard reference for Polish
The MacQuarie is published by Macquarie University of Sydney. It is considered to be the standard reference for Australian English.
Oxford English Dictionary
The OED is considered the final authority of British English, and has the oldest roots of any English dictionary. It is completely descriptive, giving all forms, spellings and meanings of all words that have occurred in English.
Merriam-Webster's Dictionary is a dictionary with very old roots. It started as Webster's Dictionary and was later acquired by the publisher Merriam, who decided to keep the Webster's brandname. Of course, Webster's 1913 is used on E2 as well
Duden Deutsches Universalwörterbuch - Duden German universal dictionary
The German government has an official spelling guideline, but publishing a dictionary is left to commercial publishers. These incorporate the guideline in the spelling of their entries. Duden is the oldest and most respected dictionary in German.
Verklarende Handwoordeboek van die Afrikaanse Taal - Explanatory dictionary of the Afrikaans Language
This is the short version of the "Woordeboek van die Afrikaanse Taal", which will be the complete dictionary for Afrikaans once it is completed.
Retskrivningsordbogen - Official Danish orthography dictionary
This is the official Danish Dictionary as published by the Dansk Sprognævn ("Danish Language Council")
Diccionario de la lengua española de la Real Academia Española - Dictionary of the Spanish Language of the Royal Spanish Academy
This is the official dictionary of Spanish, but it has been criticized for being elitist and slow to adapt changes.
Svenska Akademiens OrdLista - Word list of the Swedish Academy
This is the short, single volume form of the as yet uncompleted "Svenska Akademiens Ordbok", which started in 1898 and is due for completion in 2017. It is published by the Swedish Academy and is considered the final arbiter on Swedish spelling.
Eesti õigekeelsussõnaraamat - An Estonian ortological dictionary
Official dictionary published by the Institute of the Estonian Language. Estonian is a very small language and considered threatened by extinction. The Institute is in place to help the language survive and regulate it's use in Estonia.
Thanks go to Ariloulaleelay, La petite mort, Albert herring, koala and heisenberg for giving me information on their country's specific dictionary.
All suggestions and additions are welcomed. I do not intend this write-up to be exhaustive, and to some degree these are all subjective opinions by me or other people. Please respect these opinions, and realize that in languages that are spoken in multiple countries there will always be debate about correct spelling.
update May 18th 2008: Added Swedish and Estonian with help from Montecarlo