The Leaky Cauldron (or TLC) is, in my humble opinion, the best news site to visit for any scrap of information that is in any way remotely related to the Harry Potter world - the books, the author, the movies, the actors... etc.!
This is my cousin Nate's site, a collection of resources about - guess? - all the major languages of the world. He provides a bunch of links to sites about how to learn various other languages, as well as giving general travel advice and including links to flight booking and trip planning websites.
The Harry Potter Lexicon is the ultimate resource when you are looking up details from the Harry Potter books. They've got every little thing catalogued about the characters, the spells, the magical creatures, the layout of Hogwarts castle, and much, much more. (I've heard that J.K. Rowling herself even refreshes her memory there from time to time!)
I love this site, which contains all kinds of information about Ireland - its culture, history, geography, language, architecture... pretty much everything. Especially check out the Irish toasts and blessings; they're charming! Got some funny jokes there too.
Looking for some cool Terry Pratchett merchandise? This is the official website of Paul Kidby, Pratchett's favorite Discworld artist. The stuff here is a bit pricy, but I just love to look at it. What a cool artist....
and
www.sugarquill.net
Tired of the long wait between HP books 6 and 7? Try reading some fan fiction in the meantime! These sites are the best and largest ones I have found that house Harry Potter fan fiction stories. (If you don't know what fan fiction is, check them out, and you'll soon find out.)
People have posted some pretty awesome travel blogs here, and you can make one too! I started my Ireland trip site at Geocities before I discovered this site, and I'm not about to take the time and change over. But this site is cool all the same and looks really nice. Plus, it gives you a format, so it's easier to put together.
Related to the link above, this is my own personal page on FictionAlley. Here you can read (and review, please!!) my Harry Potter fan fic stories. Well, what I've got published so far. There's more in the works.
Gosh, how could I leave this one for so far down the page?? J.K. Rowling's official website - check it out now! It is amazingly well done! The Flash version is, anyway. All sorts of extra-textual nuggets of fascinating information are found here about Harry Potter's world, as well as several different puzzles for you to figure out and get "rewards." And on the rare occasions when her Secret Door opens... be prepared for more delights!