Mary Borsellino has created a fantastic reference page for werewolves and big, black dogs in folklore. Find it here.

English-To-Latin Translator - A search engine to help you create those handy little spells.
Dictionary of Names - An excellent source of given and surnames for magical sorts.
Archaic Occupation Names - These make good names for characters in the HP universe.
Merriam-Webster Online - The writer's best friend. Offers both a dictionary and thesaurus. Use them.
Guide to Britain For the Confused - A great resource for non-Brits on everything from money to telly.
British Words Lexicon - Another way to sound like you know what you're doing.
Dictionaries of Slang - Exactly what it says it is. Kind of fun, too.
Rough Slang - Dirty Brit-Speak. Have fun, kids, but use sparingly.
Wordsmith.org - A great little site for fans of words. You could find something useful, here.
Stars and Constellations - Everything you ever wanted to know about the bright bits in the sky.
Lunar Calendar - Want to know when the full moons were in 1976? Here's how.
Calendars Past and Present - So you know what days were which even back in the Marauders' era.
Measurement Converter - What is Farenheit in Celcius? This figures it for you.
Ghostdragon.net - A good source of info on Paganism, with some herb lore.
Witchcraft Documents - Information on the Witch Hysteria.
Myths, Fables and Folklore - Because sometimes you need a little magic.
Encyclopaedia Mythica - An incredible resource of all things legendary.
The Harry Potter Lexicon - An exhaustive source of HP fact and speculation. Not the most reliable source out there, but it'll do in a pinch.
Quidditch Teams - This'll tell you everything you need to know about the Quidditch teams in HP.
Beta Readers - Harry Potter - Please, for the love of all that is good and holy, use a beta reader.
The Akashic Record - A compendium of lore relating to Harry Potter.
Shakespearian Insult Generator - You may not need this, but it's still fun.
Bible Gateway - Search any Bible, and every Bible, for something relevant.
The Alchemy Website - Your Potions Master will like this.
Penis-isms - Wedding tackle, stiffy, bed snake; a penis by any other name...