My favorites will always be MuggleNet and The Leaky Cauldron because they brought me into the fandom with their podcasts. I know people involved with AccioPotter but it's just not the same as a site that's been around since the beginning (in my opinion). I don't know if people still use MuggleSpace or the Forums but that's where I met a majority of my HP friends. The HPA is another good site to check out because everyone seems to be involved in one way or another. Slowly though YouTube is replacing the sites since most people who founded the sites (and most wrockers) can be found there.