The most interesting resource isn't technically a fan resource. It's the "Ask Keith Baker" thread (and the condensed "Ask Keith Baker FAQ") on the WizKids Eberron boards -
http://boards1.wizards.com/forumdisplay.php?f=503
There are a lot of interesting insights into the world and Keith's views on it (which occasionally vary from WotC canon).
Next on the list would be the Korranberg Chronicle -
http://keen.datavortex.net/ . This contains issues of the Korranberg Chronicle that are
great props for a game. I like giving them out during a game and seeing what stories the players seem interested in.
Jhonen Olain's Eberron Journal has a lot of useful articles, links and information. It's been slow of late, but it's been picking up.
http://www.coveworld.net/eberron/
You can find some Eberron things at Keith Bakers site (
http://www.bossythecow.com/ ). Because of his relationship with WotC, though, he's limited in what he can put up there.
Finally, I like the Eberron Bestiary (
http://www.geocities.com/ieattrollsforbreakfast/home.html ). Some interesting monsters created for Eberron.
There are other sites out there. However, nothing else I've seen has seemed very useful to me.