Believe it or not, this isn't the only Haibane Renmei related website on the internet. There are plenty other sites that are interesting, informative, and helpful. I've tried to put together a list of some of the best so you don't have to spend too much time wandering around Google. If you have a site and you'd like to be linked, feel free to e-mail me about it.
Official Haibane Renmei Site
This site is pretty awful but since it is the official site I felt obliged to link it. There's not much here except a couple of video clips and some lame character profiles.
Official Haibane Renmei Site
Not much use to you unless you speak Japanese but it's still pretty interesting to wander around.
Sekai no Hajimari This site is an excellent resource to anyone interested in Haibane. I learned most of what I know about the show from it's forum: the Old Home Bulletin Board. There is also an extensive fan works section with many contributions from the OHBB community.
The Guestroom Another forum. When I last checked it was just getting started but it looks to have a lot of potential, be sure to check it out.
Within the Wall This is a very helpful website and actually helped me make the series "click" in my head. Be sure to pay a visit to this site and look around.
Greywing: A Haibane Renmei Fansite An informative site with character profiles, episode summaries, and more.
Haibane Renmei News This site is in French so you may need Babelfish but even if you don't understand it this site is pretty cool. It's got some nice desktops and "goodies".
Old Home A cool site with a lot of useful information.
Which Haibane Character Are You? A cheesy little quiz but it's fun. I got Hikari!
Charcoal Feathered Wings This is Ran-chan's site. A few coding issues but still a cool site. It's got a very nice little page about yoshitoshi ABe himeself.
How to Make Haibane Wings Guide Taters' awesome guide on how to make Haibane wings for cosplay.
Haibane Renmei Shrine A solid little shrine with a lot of neat stuff like Winamp skins, not to mention character profiles and such. Also has a nice fanart section with links to some great fanart.
Anime Scores I got a couple of pieces of Haibane sheet music from this site. They've got lots of other shows so be sure to check it out.
Topazstorm This is Topaz's website, where I got most of the sheet music. He's got a pretty snefty site so be sure to pay it a visit.
Evangelion Eclipse While not actually related to Haibane Renmei, this site looks pretty good. As of 06/06/15 it's still under construction, but is definately worth a visit, especially if you're a fan of the classic anime Neon Genesis Evangelion.
Pixelated Noise An up and coming website that will soon have some fantastic recordings of Haibane music played on piano!