J. K. Rowling.com - This is JKR's own website, where she has news, information, and all of that good stuff.  Most of it's covered in the Lexicon, but not all.

Official Harry Potter Website - This is Warner Brothers' HP website.  There's a Sorting hat quiz and a wand picker.

Freesearch UK -  UK based dictionary...great for finding definitions of British words, and determining if a phrase is primarily from the US (this site specifies).

A Dictionary of Slang - Here is a good site for British slang.  It has dozens of words, and even the area where each word is usually used.

British Dialect Translator - I don't know how accurate this one is, but it's a lot of fun and can offer some insight as to how some dialects might look in type.

Metric Converter - Converts measurements into metric.

Magic Properties of Wood - This is a cool site which tells of the various magical properties of wood, according to Wiccan tradition.

The United Kingdom on Wikipedia - All sorts of UK information.

Writing Tips - This is a website (my own) of general writing tips of use to beginners in particular, but also good as a refresher course.

Merriam-webster.com - The official M-W website, you can find definitions for just about anything.  There's a thesaurus, too.

Dictionary.com - Another dictionary site, with a thesaurus, web-search, and encyclopedia.

Ms. Nitpicker - This is a former English teacher who is full of very good, albeit a bit sarcastic advice on writing.