The Best of Sites
Because I am unable to provide what all (or most) Harry Potter fans want (a interactive RPG where they can BECOME a hogwarts student, in their mind at least) instead i will provide a list of what I think are the best Harry Potter sites. Remember two things when reading the list: 1) This is my opinion and I still have full respect for ALL Harry Potter sites, 2) This is only what I have discovered so far... There may be many other great sites waiting to be looked at, so keep searching!
1. Hexrpg - Writers Choice
This might be my all time favorite site. It balances Harry Potter with real life, fun with rules, and makes it a great place for people of all ages to enjoy their favorite places in the wizarding world including Hogsmede, The Lake, Hogwarts Express, and your common room according to what house you are placed in. Unlike most Harry Potter RPG sites to be placed in a house you type a letter through "owl post" to the administrators with your characters personality and traits. Other features include being able to open your own wizard shop and sell goods to earn gallons and (or course!) play lots of RPG while you make new friends.
2. Xauror (ex-auror)
This was one of my first RPG Harry Potter sites I discovered that made a strong impression on me. Like Hexrpg and others it's basis is on interaction between members. You meet many other Harry Potter fans (like you will at any Harry Potter site) and your RPG character can gain friends and relationships with others characters. You can even start dating, get married, or run away together, and adopt children, or be adopted. Unfortunately this all cost gallons, and in this world gallons are hard to get!
3. Harrypotterhaven
I personally have not have much time on Harrypotterhaven but from the brief looking around i have been able to do it looks very good. Again like many sites, harrypotterhaven lets you go to different areas of the harry potter world and respond to (or create) posts. It is not a 3d universe but with the level of detail some people put into what they write it almost feels like it. You can keep up with what is happening in their world (or get up to date if you just join) by reading their Daily Prophet. You can also become a student OR staff member/adult in the world. This opens up many more possibilities to improve or change any current characters on any other sites. Overall I feel like it is very well organized and pulled together. I recommend to take a look (ever if you choose not to join).
4. JK Rowling Official Site
It is always good to get the authors point of view on the story, and that is given and MUCH more! You can read rumors and see if they are true or false, get links to fan sites, extras, Q&A, and her personal biography. My personal favorite part is the Weasly family tree. To reach it you click the pink eraser on the home page. When the page loads click the door to open, then finally click the piece of paper with the dark mark on it.
5. Chamber of Chat
This really is a 3D chat space. Unfortunately it is not the most advanced in members or things to do.
6. Beauxbatons Academy of Magic
This newly opened site (as of Feb. 25) is much like the Hogwarts RPGs except taking place at Beauxbatons. As they say on the homepage "Mind you, we're a new site, so we're still working through all the kinks and such 
This also explains why the site looks so empty... we're not inactive...". It is new but you can tell it is going to be a success. They have the framework laid down but it is not advanced yet. Here is a bit of information they offer in a early post/greeting message: "This is a non-canon Harry Potter RPG, currently set in the year 1989. As a newly opened site, we have a variety of things to offer, from joining the four unique houses and competing for the House Cup, to roleplaying with fellow members."
7. Darktimesrpg
When I came across this site in my Google searches it struck me with similarity to my personal favorite site (hexrpg). As I continued looking through the site I liked what I saw. I was well organized and full of information. It was interactive yet not so "everything is post" format like you find in some. However when I entered the site there where 0 users online. I believe this will change during the day and on weekends. But it sill made me sad to see that such a great site had so little users.
8. Mugglenet
This site is not so much a RPG but a source for information about the movies, books, and much more! Features include mistakes (one of my main resource for my mistakes page) and name origins.
9. Veritaserum
Like its name it tells the truth about the movies, books, actors, etc.
10. HP-Lexicon
This site is more of a information guide to the Harry Potter world than anything else. However it does have lots of things to look at and it kept me entertained for a long period of time.