Webster's Online Dictionary and Multilingual Thesaurus
Webster's Online Dictionary has the avowed goal of creating the largest online dictionary in the world, available currently in 90 modern and 10 ancestral languages. Part of its source material is also based on the 1913 Webster's Dictionary. They do not worry whether a word is slang or profanity, but do define it as such.
Noah Webster does have a online dictionary presence now.
It is certainly the most comprehensive of all the free dictionaries online. It includes the definitions for a word, the date the word was first used in popular English literature, the etymology of the word in several languages, if relevant, including the word in different root languages and specialty definitions of the word in various domains. It has additional specialty definitions of the word with context, and the word used as part of a phrase in different disciplines. It offers Abbreviations, Acronyms, Synonyms, Synonyms within context, as in a thesaurus, and a section on crosswords, where it gives words which may be used with the definition of the word you are defining. It gives the languages where the same word is used, and the meaning of the word in that language.
There is a section on modern usage, with the word used in screenplays, lyrics, and movie titles. There is a section on commercial usage, with a link to the commercial site. It also gives images of the word in photos, illustrations, and digital media, and sound files captioned with the word. There are also sections on the use of the word in both fiction and non fiction usage, spoken usage, and speeches. As well, the usage frequency of the word in the English language is provided, along with the part of speech most often used, and the percentages of other parts of speech in which the word is used. It provides expressions where the word is used and the frequency of Internet keywords using the word daily.
Modern translations of the word are provided in 40 different languages in the word I researched, along with alternative meanings in that language, as well as ancestral language translations for the word. It gives a Bible trace for the word, with the source and the word in the original edition of the Bible, and the same Bible verse given in a number of other languages. There are derivations and misspellings, along with most of the common misspellings which bring you the word as a choice in a spellchecker. There are words which rhyme with the word you selected, and anagrams using the letters in the word.
At the end, there are sections with the word used in titles for literature, art, professional photos, proper noun and trade noun usage, music, videos, and products. These last sections all contain links to where you can purchase the products, and affiliate income from these purchases help maintain the site.
Finally, it gives a Google search box with the word, and an index with links to all the sections in the word definition in the Webster's Online Dictionary.
It has instructions for putting on a search box for the dictionary on your own website.