Official Horizons Site This is the Artifact Entertainment site and contains all official releases.
Crossroads of Istaria This is one of the best Horizons sites and is home to the Istaria times and the largest irc HZ chat. This site updates it's news and chat logs quickly.
Horizons Vault This is another good site, and is part of the Vault/ign network. It hosts the major BB system and also has a lot of exclusive info on the site. This site updates its news frequently.
Horizons Another major site, with extensive message boards and a large unofficial FAQ with quotes from Dave.
Encyclopedia Istaria This site has a massive amount of information on HZ, all extremely well organized. It also has a great section of 3D renderings of Istaria, for all the people who can't wait for release.
Can I be in beta? Maybe the best Horizons humour site out there, this site has such sections as Gnome of the Week and various rants. A great place to relax and get away from the gnomes on the Vault boards.
Dragonspires of Istaria This is definatly a dragon centered Horizons page, but it has a partially dragon centered Bulletin Board and the news page is updated frequently.
The Horizons Palace Kind of empty.
Angels of Istaria This site is centered around the angels and great deal of information on angels as well as a good news page.
Horizons Haven This is another great site and has a lot of information on Horizons, a great news page and hosts 2 guilds for Horizons.
Horizons Hub This is a good news sites and has a lot of links, though they are put in with the menu and could use bullets to make it more readable. It also has a nice section titled "What's Horizons?" which provides a good overview.
Magic of Istaria Looks promising, but so far none of the sections contain any information and the news page could be updated more.
Dwarves of Istaria The page has a lot of info on the dwarves in Horizons and the newspage stays up to date with all the news released about the dwarves.
Horizons Germany It's in German so I can't really say that much about it, but it looks good.
The Diary of an Adventurer This is Ianash's art and story site. It has a fair amount of his art and stories on it.
Dwarves in Horizons This site is part of the Righteous Earthfolk's site and has a lot of information on Dwarves.