For the linguistically enthusiastic:
The single most fantastic multilingual dictionary I've found to date.
The Speech Accent Archive
Free access to literally hundreds of voice recordings of speakers from all over the world. If you've never heard an Antiguan accent, or want to listen for the differences between Russian and Tatar speakers, you can't beat this. NB: all speech is in English.
Latin Dictionary and Grammar Aid
Fabricati Diem, Punc.
The Old English Dictionary
Its milkshake brings all the Geats to the mead hall, despite not having a word for 'milkshake'.
Online Etymology Dictionary
If you're not sure whether "satellite" was in common use in 1854, or would like to know whether "proctor" and "proctology" have the same root, this is your Holy Grail.
Baby Name Finder
Do you need a two-syllable Danish boy's name? A Hawaiian name ending in I? Dial one up here.
Kate Monk's Onomastikon (Dictionary of Names)
The question isn't "can you look up place- and period-accurate names online?" The question is, "do you want your Faroese name to have a Norse, Germanic, Classical, or Biblical origin?"
She fights bad grammar like Imodium fights gas: gently, and without judging you.
For the American history buff:
The Inflation Calculator
Just what it says on the tin: calculate price changes from any year between 1800 and 2012.
An absolutely depressing amount of information and photos of glass bottles, from 19th to mid-20th centuries.
Over a thousand historical photographs of indigenous Americans. For God's sake, people, don't take George Catlin's word for it.
The Quack Doctor
An exhaustive all-you-can-stomach index of the antiphlegmatics, neuro-sanative elixirs, and rectal dilators of yesteryear.
Western Slang and Phrases
Dish up a bowl of Pecos strawberries and get reading - this dictionary is HUGE.
The staggeringly comprehensive Catholic encylopedia.
Ram's Horn Studio
Put the gas in your gaslamp fantasy: this site shows off more lanterns, stoves, lights and candlesticks than you can shake a stick at. Well, you can, but it will catch fire. Period-accurate fire.
More to come!