I wanted to supply you all with information, but why bother when there are huge sites out there that already do just that? Below is a list of sites to help, but from me... here's my spellbook!

info | writing

Encyclopedia Potterica

Did you read the title? ^-^

Harry Potter - The Ultimate Guide

Lots, lots of useful information can be gathered from here dealing with the world of Harry Potter. Check it out!

MuggleNet

Before you go checking out this gigantic site, for specific info, check out their Facts page. However there are 2 sections of this site that I highly recommend:
Ministry of Magic
Whole listing of each department and who the head and members are.
Hogwarts
Can I say very helpful? This is a must. Has an extensive list of Hogwarts teachers, library books, and history of the four houses.

What's in a name?

A dictionary, literally, of what each and every Harry Potter name means and originated from. Quite impressive and very interesting. (Run by Priscilla Spencer)

Harry Potter Word Origins

If the above link didn't fill your appetite, then here's another one. Related words and obscure meanings for names and terms in the books by Gwillim Law.

Harry Potter Pronounciation

Want to get the names right? Here you go.

Writers University (must visit)

I highly recommend you go here whether you need help or not. A whole list of beta-readers who are good at what they do and tons of other things to help you improve in your writing. Mary-Sue test, etc.

Tips, Tools, and Ideas to Improve your Writing

Doesn't focus on fanfiction writing, but rather writing in general. Nice bits of pointers that could help you.

Fanfiction.net's Beta-Reader Forum

Post a topic for a beta-reader. I suggest, however, to go to Writer's University first. That is just a suggestion.

Care to add another link? Then please mail it to me