Dictionaries!

In no particular order:

1736 Canting Dictionary
A Collection of the Canting Words and Terms, both ancient and modern, used by Beggars, Gypsies, Cheats, House-Breakers, Shop-Lifters, Foot-Pads, Highway-Men, &c. Taken from The Universal Etymological English Dictionary, by N. Bailey, London, 1737, Vol. II, and transcrib'd into XML Most Diligently by Liam Quin.

The ABCs of Diplomacy (PDF!)
Compiled by The Swiss Federal Department of Foreign Affairs (DFA)

ASCII Art Dictionary

Ballet Dictionary
Dover Publications has graciously allowed the use of 170 terms from the Technical Manual and Dictionary of Classical Ballet, which are then demonstrated by ABT Company dancers.

The Devil's Dictionary by Ambrose Bierce
DICTIONARY, n. A malevolent literary device for cramping the growth of a language and making it hard and inelastic. This dictionary, however, is a most useful work.

Dictionary of All-Consonant Words
Dictionary of All-Vowel Words

Dictionary of Difficult Words
Do you aim to become a member of the literati, or do you wish to be a savant? Do you want to avoid being verbigerative and be succinct instead?

Samuel Johnson's Dictionary of the English Language (Google Book!)
I knew that the work in which I engaged is generally considered as drudgery for the blind, as the proper toil of artless industry; a task that requires neither the light of learning, nor the activity of genius, but may be successfully performed without any higher quality than that of bearing burdens with dull patience, and beating the track of the alphabet with sluggish resolution. From The Plan of an English Dictionary

Bartleby's Dictionary of Phrase & Fable
...over 18,000 entries that reveal the etymologies, trace the origins and otherwise catalog "words with a tale to tell."

Flaubert's Dictionary of Received Ideas
DICTIONARY Say of it: 'It's only for ignoramuses!' A rhyming dictionary?-- 'I'd rather die than use one!'

The Difference Dictionary
Dr. Gunn's Organic History Supplement for The Difference Engine, a novel by William Gibson and Bruce Sterling.

DoD Dictionary of Military Terms
PEACEMAKING (DOD) The process of diplomacy, mediation, negotiation, or other forms of peaceful settlements that arranges an end to a dispute and resolves issues that led to it.

The Double-Tongued Dictionary
A lexicon of fringe English, focusing on slang, jargon, and new words.

Field Guide to Identifying Unicorns by Sound
Not exactly a dictionary, but definitely a good reference to keep handy.

The Foolish Dictionary
An exhausting work of reference to un-certain English words, their origin, meaning, legitimate and illegitimate use, confused by A FEW PICTURES

The Grandiloquent Dictionary
This is the result of an ongoing project to collect and distribute the most obscure and rare words in the English language. It also contains a few words which do not have equivalent words in English.

Cab Calloway's Hepster's Dictionary
Some six years ago I compiled the first glossary of words, expressions, and the general patois employed by musicians and entertainers in New York’s teeming Harlem. That the general public agreed with me is amply evidenced by the fact that the present issue is the sixth edition since 1938 and is the official jive language reference book of the New York Public Library.

The Nonverbal Dictionary of Gestures, Signs, & Body Language Cues
From Adam's-Apple-Jump to Zygomatic Smile

The Not-So-Correct Dictionary

OEDILF - The Omnificent English Dictionary In Limerick Form

The Online Etymology Dictionary
...a map of the wheel-ruts of modern English.

Online Dictionary of Symbolism
This symbolism dictionary endeavors to provide some possible cultural significances of various symbols, and suggest ways in which those symbols may have been used in context.

Kate Monk's Onomastikon
(Dictionary of Names)